Waking up at 3am and not being able to fall back asleep is one of the most frustrating parts of bad sleep. I've spent the past year reading through research studies on sleep, and most of it points back to daily habits. I cut out the stuff you can't change (age, sex, menopause, sleep apnea) and ranked what's left by how big the effect was and how easy it is to change.

Biggest effect. Most controllable.

1. Cut caffeine earlier than you think. Caffeine added ~12 min of nighttime waking in a 24 study meta-analysis. If you're waking up at 3am and drinking coffee after lunch, start here.

2. Regular moderate cardio. Cut ~10 min of nighttime waking in a meta-analysis of RCTs in people with insomnia. Evening workouts are fine. Don't go hard within 1 hour of bed, that's the one window where it can fragment sleep.

3. Make the room actually dark. Even dim light (5 to 10 lux, roughly a nightlight) increased awakenings in a PSG study. Blackout curtains or a sleep mask.

4. Block the noise. In one rail noise study from the WHO review, noise added ~30 min of nighttime waking. Earplugs or a fan for white noise if you live anywhere loud.

5. Keep the bedroom cool. Keep it under at least 77°F (25°C). Above that, nighttime waking goes up, and a room that's too cold wakes you up too.

Often overlooked

6. Stop eating 1+ hours before bed. Eating within an hour of bed showed 2 to 2.6x higher odds of waking up during the night.

7. Open a window or run a fan for airflow. CO2 at 1000 ppm (a closed bedroom can get there fast) added ~5 min of wake time.

8. Skip the nightcap. Alcohol before bed was linked to lower sleep efficiency, meaning more of your time in bed spent not actually asleep.

9. Add strength training. A review of 13 RCTs found resistance training improved mid-sleep disturbance and sleep efficiency.

Smaller or more nuance

10. More fiber, less sugar. More fiber was linked to fewer arousals and deeper sleep. More sugar was linked to more arousals. Small study (n=26) but tightly controlled.

11. Tart cherry juice. Cut nighttime waking by ~17 min vs placebo, small study as well.

12. Yoga. Showed a huge reduction in waking (~56 min) in insomnia patients, but the evidence is rated low certainty so the real effect is probably smaller.

Supplements

Short version on supplements if you've made it this far.

• Standard melatonin doesn't help you stay asleep. It's for timing, not maintenance
• Ashwagandha had the best data for reducing nighttime waking
• Glycine reduced waking, but in a very small study
• Magnesium improved sleep efficiency in older adults but never measured waking directly
• Valerian made people feel like they slept better, with no measured change

Wake ups throughout the night cost you a piece of the night's recovery. So here's a guide I built to better understand what influences staying asleep and focuses mostly on Wake After Sleep Onset. I broke this into 5 main areas: nutrition, supplements, exercise, environment, and demographics. There's a plain english explanation column for each row/short definitions to try to help make it easier to understand each one briefly. All sources are linked in the third column as I hope this guide makes it easier for you to do your own research into each factor further.

Thank you all for being so receptive to my last post! I have been posting these in other communities over the last year so I'll try to share one here once a week. I know tables aren't ideal on mobile, so I'll include a mobile friendly version of this resource in the comments

Acronyms:
WASO - Wake After Sleep Onset
PSG - Polysomnography
SMD - Standardized Mean Difference
RCT - Randomized Controlled Trial
SWS - Slow Wave Sleep
AHI - Apnea-Hypopnea Index
SWSD - Shift Work Sleep Disorder
HPA - Hypothalamic-Pituitary-Adrenal (axis)

Nutrition

Factor	Impact	Key Info (study/effect size)	Plain English	Evidence
Dietary Fiber	Decrease Arousals	St-Onge 2016 n=26, PSG, controlled crossover	More fiber = fewer nighttime wake-ups	Strong
Sugar / Refined Carbs	Increase Arousals	St-Onge 2016 n=26, PSG, significant predictor	Sugar directly increased sleep arousals	Strong
Caffeine	Increase WASO +12 min	Gardiner 2023 meta-analysis, 24 studies	Caffeine adds ~12 min of nighttime waking	Strong
Alcohol	Increase Fragmentation	Spadola 2019 Jackson Heart Study, n=785, actigraphy	Sleep breaks apart as alcohol metabolizes	Strong
Late Eating (<1hr)	Increase WASO 2–2.6× odds	Crispim 2022 British J Nutrition, large n	Eating right before bed doubles wake-ups	Moderate
Tart Cherry Juice	Decrease WASO ~17 min	Pigeon 2010 n=15, RCT crossover, insomnia cohort	Cherry juice cut nighttime waking vs placebo	Moderate

Supplements

Factor	Impact	Key Info (study/effect size)	Plain English	Evidence
Melatonin (immediate-release)	No significant WASO effect	Moon 2022 meta-analysis of RCTs, Neuropsychopharmacology	Standard melatonin does not help you stay asleep	Strong
Ashwagandha (600mg/day)	Decrease WASO, SMD −0.39	Cheah 2021 meta-analysis, 5 RCTs, n=400 (3 trials/281 for WASO)	Ashwagandha reduced nighttime waking	Strong
Glycine (3g)	Decrease WASO, faster SWS onset	Yamadera 2007 n=11, PSG-measured, crossover	Glycine reduced waking and deepened sleep	Moderate
Magnesium (500mg)	Increase Sleep efficiency (elderly)	Abbasi 2012 RCT, n=46, 8-week, 65+ years	Improved efficiency but no direct WASO data	Limited
L-Theanine (200–450mg)	Mixed WASO results	Cotter 2025 benefits at 200–450mg/day	Some maintenance benefit but inconsistent alone	Limited
Valerian Root	No consistent WASO benefit	Shinjyo 2020 meta-analysis, 60 studies, n=6894	Subjective improvement only, no objective WASO change	Weak

Exercise

Factor	Impact	Key Info (study/effect size)	Plain English	Evidence
Moderate Aerobic Exercise	Decrease WASO ~10 min	Riedel 2024 meta-analysis of RCTs, insomnia patients	Regular cardio cuts ~10 min of nighttime waking	Strong
Resistance Training	Decrease Sleep disturbance, Increase efficiency	Kovacevic 2018 systematic review, 13 studies, n=652	Strength training improved mid-sleep disturbance	Moderate
Yoga	Decrease WASO ~56 min	Bu 2025 network meta-analysis, 22 RCTs, n=1348	Yoga showed large WASO reduction in insomnia patients	Low
Evening Moderate Exercise	Decrease WASO	Dolezal 2017 systematic review, 34 studies	Moderate evening exercise helps you stay asleep	Moderate
Vigorous Exercise ≤1hr Before Bed	Increase WASO risk	Stutz 2019 meta-analysis, 23 studies, Sports Medicine	Intense exercise right before bed may fragment sleep	Moderate

Environment

Factor	Impact	Key Info (study/effect size)	Plain English	Evidence
Bedroom Temp (20–25°C)	Decrease WASO at optimal range	Akiyama 2021 PSG-measured, 20–25°C optimal	Too hot or cold increases nighttime waking	Strong
Light at Night (even dim)	Increase WASO	Cho 2016 n=23, PSG, 5–10 lux, Chronobiology Int	Even dim light during sleep increases wake time	Strong
Noise (>50 dBA)	Increase WASO +30 min	Basner 2018 WHO systematic review, 74 studies	Noise above 50 dB adds ~30 min of waking	Strong
CO2 >1000 ppm (poor ventilation)	Increase Wake time +5 min	Kang 2024 n=36, field-lab, 3 ventilation levels	Stuffy bedroom air measurably fragments sleep	Moderate
Mattress (medium-firm)	Decrease Most consistent WASO	Hu 2025 n=12, PSG, 3 firmness levels compared	Medium-firm mattress gave most stable sleep	Limited

Demographics

Factor	Impact	Key Info (study/effect size)	Plain English	Evidence
Aging (30–60+)	Increase WASO ~10 min/decade	Ohayon 2004 meta-analysis, 65 studies, 3,577 subjects	Each decade adds ~10 min of nighttime waking	Strong
Female Sex	Paradox: more complaints, better PSG	Ohayon 2004 women better objective metrics, more subjective complaints	Women report worse sleep but objectively sleep better	Strong
Menopause (hot flashes)	Increase WASO, 69% flashes  awakening	Joffe 2013 PSG + GnRH model, n=29, hot flashes = 27% of WASO	Nighttime hot flashes are a major wake trigger	Strong
Obesity (BMI ≥30)	Increase WASO significantly	Zhao 2021 Sleep Heart Health Study, n=5,723, PSG	Higher WASO independently associated with obesity	Strong
Shift Work	Increase WASO, Decrease sleep efficiency	Wickwire 2017 narrative review, SWSD patients, Chest	Shift workers have more fragmented daytime sleep	Moderate
Nocturia (≥2 episodes)	Increase WASO +34 min	Fung 2017 SOF study, n=1,520, actigraphy	More bathroom trips = much more nighttime waking	Strong
Obstructive Sleep Apnea	Increase WASO, Increase arousals with severity	Patel 2019 comprehensive review, PSG data	Each breathing event triggers arousal and waking	Strong
Chronic Pain	Increase WASO, large effect	Mathias 2018 meta-analysis, 37 studies, PSG	Pain significantly increases nighttime wake time	Strong
Psychological Stress	Increase WASO via cortisol elevation	Vgontzas 2001 n=24, 24-hr cortisol sampling, PSG	Stress hormones directly fragment sleep	Moderate

Resting heart rate is one of the simplest and most consistent markers of cardiovascular fitness. So here's a guide to better understand what influences RHR, including a few supplements that tested out with no significant effect (figured they were important to include). I broke the data into five main categories: nutrition, supplements, exercise, environment, and demographics. I added a plain English breakdown in the 4th column to make it easier to skim, and all sources are linked in the key info column so you can further your own research. Hope this helps!

Thank you again to everyone who has commented on the last two posts I did! Really appreciate it. Luckily I wrote these guides over the last year so I am able to share about 1 a week for a few more weeks to this community. I'll include a mobile friendly version of this resource in the comments as well as the tables aren't great for mobile readers.

Exercise

Factor	Impact	Key Info	Plain English	Evidence
Endurance / Aerobic Training	Decrease RHR	Huang 2005 meta-analysis older adults, SES −0.58	Cardio = −8.4% RHR; >30 wk = −8.37 bpm	Strong
HIIT	Decrease RHR	Edwards 2023 meta-analysis 97 RCTs, n=3,399	HIIT lowers RHR by ≈3.9 bpm	Strong
Yoga	Decrease RHR	Cramer 2014 meta-analysis 32 trials	Yoga = −5.27 bpm vs non-exercise	Strong
Cardio Fitness Level	Decrease RHR	Gonzales 2023 Fenland Study, n=10,865	Fitter people have lower RHR	Strong
Resistance Training (alone)	No effect	López-Valenciano 2019meta-analysis 16 trials	Lifting alone barely changes RHR	Strong
Slow Breathing / Breathwork	Decrease RHR	Garg 2024 −1.72 bpm	Daily breathing exercises lower RHR	Strong
Tai Chi / Qigong	Decrease RHR	Reimers 2018 meta-analysis subgroup	Mind-body exercise lowers RHR	Moderate
Sedentary Behavior	Increase RHR	Hallman 2019 n=490 workers, nocturnal HR	More sitting raises RHR	Moderate
Overtraining	Increase RHR	Bosquet 2008 systematic review	Excess training raises RHR	Moderate

Nutrition

Factor	Impact	Key Info	Plain English	Evidence
Omega-3 (EPA/DHA)	Decrease RHR	Hidayat 2017 meta-analysis 51 RCTs, n≈3,000	Fish oil slows RHR by 1.6–2.5 bpm	Strong
Coffee	No effect	Coffee meta-analysis 2023 6 RCTs, n=485	Daily coffee barely changes RHR	Strong
Acute Caffeine (>3 mg/kg)	Increase RHR	Mesas 2014 review of caffeine BP/HR	Big caffeine dose raises RHR	Moderate
Alcohol	Increase RHR	PLOS Digital Health 2026 n=20,968, 5.1M person-days	One drink = +2.4–2.8 bpm overnight	Strong
Mediterranean Diet	Decrease RHR	García-López 2014 SUN cohort n=15,863	High vs low adherence = −2.2 bpm	Moderate
Reduced Sodium	Increase RHR	Graudal 2016 meta-analysis 63 RCTs	Cutting salt nudges RHR up 1.65 bpm	Strong
Smoking / Nicotine	Increase RHR	Carta 2015 n=141,317: +0.21 bpm per cigarette/day; cessation drops 5–15 bpm within a day	Smoking raises RHR; quitting drops it fast	Strong

Supplements

Factor	Impact	Key Info	Plain English	Evidence
Oat Bran Fiber (HTN)	Decrease RHR	Nutrients 2022 RCT n=70, 3 mo, DASH-controlled	Oat bran lowers RHR in people with high blood pressure (HTN)	Weak
Chromium (MetS)	Decrease RHR	PubMed 28856601 RCT in MetS/IGT	Chromium lowered RHR in metabolic syndrome only	Weak
Potassium	No effect	Gijsbers 2016 meta-analysis 22 RCTs, n=1,086	Potassium pills don’t change RHR	Strong
Vitamin D	No effect	BEST 2017 RCT n=305, 12 months	No RHR effect at any dose	Strong
L-Arginine	No effect	Rector 1996 RCT in HF + multiple trials	No consistent RHR change	Moderate
Melatonin	Mixed	Tobeiha 2022 review with HF safety signal	Mixed effects; emerging concerns	Weak

Environment

Factor	Impact	Key Info	Plain English	Evidence
Extreme Temperatures (hot or cold)	Increase RHR	Madaniyazi 2016 cohort n=47,591	Both hot and cold raise RHR +0.11 bpm per 1°C increase	Strong
High Altitude (acute)	Increase RHR	Bärtsch 2007 Circulation review	Going to altitude raises RHR; body adjusts in weeks	Strong
Air Pollution	Increase RHR	COPD cohort 2025 54,487 hourly observations	+2.01 bpm per IQR NO2	Moderate
Nighttime Traffic Noise	Increase RHR	Griefahn 2008 lab study n=24	+9 bpm without awakening, +30 bpm with	Strong
Time of Day	Mixed	Boudreau 2012 circadian rhythm	Peaks ~16:36, trough at night	Strong
Season (winter vs summer)	Increase in winter	Wearable cohort 2020 n=92,457	~2 bpm higher in winter	Strong

Demographics

Factor	Impact	Key Info	Plain English	Evidence
Age	Decrease RHR	Avram 2019 Health eHeart, n=66,788	−1.47 bpm per 10 years	Strong
Gender	Increase RHR	Gillum 1988 NHANES national sample	Women +6 to +14 bpm vs men	Strong
Genetics	Intrinsic	Jensen 2018 Danish twins n=4,282	~23% of RHR variance is genetic	Strong
Obesity	Increase RHR	J-shape 2023 PubMed 37249904	Higher BMI = higher RHR	Strong
Pregnancy	Increase RHR	Sanghavi 2014 Circulation review	RHR rises 10–20 bpm, peak in 3rd trimester	Strong
Menstrual Cycle (Luteal)	Increase RHR	PMC9572726 n=49 women	+2.33 bpm in luteal vs follicular	Moderate
Race / Ethnicity	Mixed	Gillum 1988 NHANES	Small race-related variation	Moderate

Quick FYI incase you have a question on these:

- For Ashwagandha the RCTs show HRV improvement and stress related changes but no significant change in resting HR. 

- For Magnesium BP effect is documented but RHR effect wasn't.

I understand there's varying opinions on the importance of an individual's daily VO2 max number from Whoop. Personally how my VO2 max trends over time is what is more interesting to me as I find it a good signal to track long term health/longevity and how much overall fitness you're gaining or losing over time.

If this is a topic you're interested in learning more about I've built out this list of factors that influence it as a starting point for anyone to learn more and do their own research. To make this easier I've broken this down by category, included a short plain english definition, and linked all sources (scroll to the right on the tables and you'll see the hyperlinks). I've removed a few from here as it was getting far too long and sorry I know the tables aren't great on mobile but I'll include the full resource in the comments.

Exercise

Factor	Impact	Study/effect	Plain English	Evidence rank
HIIT (long interval, high volume)	Increase	Wen 2019. Meta, 53 RCTs. SMD 0.50 to 2.48 vs control, 0.65 to 1.07 vs steady cardio	Intervals of 2 min+ are the only version that beats steady cardio	Strong
Detraining	Decrease	Zheng 2022. 21 athlete studies. Under 30 days ES −0.62 (−3.93%). Over 30 days ES −1.42 (−9.43%)	Under a month off costs about 4%, longer about 9%	Strong
Moderate continuous cardio	Increase	Wen 2019. Overall SMD 0.41 to 1.81 across populations	Steady cardio works, it is just less time efficient	Strong
Sprint interval training	Increase	Sloth 2013. 19 studies, 13 with VO2max data. g = 0.63 (0.39 to 0.87), +4.2 to 13.4%	Very short all out sprints reliably raise it	Moderate to Strong
Resistance training (alone)	Increase	Smart 2022. 37 studies, 22 pooled. +1.89 mL/kg/min (1.21 to 2.57) under 24 weeks. Null beyond 24 weeks	Small bump in over 60s, and only in shorter programs	Moderate

Nutrition & Supplements

Factor	Impact	Study/effect	Plain English	Evidence rank
Dietary nitrate / beetroot	No effect	Gao 2021. Meta, 73 studies, n=1,061. Submaximal VO2 −0.04 L/min (p<0.00001). VO2max not significant	Makes a given pace cheaper to hold. It does not raise your ceiling	Strong
Creatine	Slight decrease	Gras 2023. Meta, 19 RCTs, n=424. ES −0.32 (−0.51 to −0.12)	Adds body water, so the per kilo number dips slightly	Strong
Iron (when deficient)	Increase	Pasricha 2014. 24 RCTs, 18 for this figure. +2.35 mL/kg/min (0.82 to 3.88)	Fixing low iron raises it. This is correcting a deficit, not a boost	Moderate to Strong
Caffeine	No effect	Brietzke 2017. RCT, n=9. Time to exhaustion +18.7%, peak power +13%, VO2max unchanged	Helps you push harder. Much of the benefit is placebo	Moderate

Demographics

Factor	Impact	Study/effect	Plain English	Evidence rank
Body fat / excess weight	Decrease (per kg)	van Baak 2021. 66 RCTs, n=3,964. Training +3.82 mL/kg/min (3.17 to 4.48)	The score is per kilo, so losing fat raises it arithmetically	Strong
Age	Decrease	Fleg 2005. Baltimore Longitudinal Study, n=810. 3 to 6% per decade in the 20s and 30s, over 20% from the 70s	Decline accelerates with age, and faster in men from the 40s onward	Strong
Genetics (trainability)	Variable	Bouchard 1999. HERITAGE, n=481 across 98 families. Maximum heritability estimate 47%	Same training, wildly different response. It runs in families	Strong
Dehydration	Decrease	Cheuvront 2010. Mechanism review. Threshold is over 2% of body mass	Less blood volume to pump, and heat makes it worse	Moderate
Sex	Variable	Santisteban 2022. Review. 10 to 12% performance gap. Elite women about 10% lower per kg	Mostly oxygen delivery, meaning haemoglobin mass and heart size	Moderate
Immobilization / bed rest	Decrease	Saltin 1968 n=5, with McGavock 2009 follow up. −26% in three weeks	Three weeks flat on your back costs about what 40 years of aging does. Crazy.	Moderate

Lifestyle & Environment

Factor	Impact	Study/effect	Plain English	Evidence rank
Sedentary behaviour	Decrease	Eriksen 2016. n=16,025. No association among the highly active	Matters most if you are not already training hard	Moderate (observational)
Sauna bathing (habitual)	Increase	Kunutsor 2024. 2,012 men. +0.30 mL/kg/min per weekly session. Lee 2022 RCT +2.7 (0.2 to 5.3)	A small extra bump stacked on top of training	Moderate
Smoking	Decrease	Caci 2025. n=70, step test estimate. 38.8 vs 41.6 in never smokers	Lower aerobic ceiling. Whether quitting reverses it is untested here	Moderate
Altitude (live high, train low)	Increase	Wehrlin 2006. 10 elite orienteers at 2,500 m for 18 h/day over 24 days. VO2max 3,515 to 3,660 mL/min	Sleeping high builds more oxygen carrying red cells	Moderate
Heat acclimation	Increase	Waldron 2021. 28 articles. Hedges' g 0.42 cool, 0.63 hot. Lorenzo 2010 +5% cool, +8% hot	Plasma volume expands. Clearest when you are also tested in heat	Moderate

I wanted to try to identify which brand out of Fitbit, Oura, Apple Watch, Whoop and Garmin has the "most accurate" sleep tracking across different sleep metrics. Well, 14 sleep lab (polysomnography) studies later, here's a summary of what I found for Fitbit.

Short version: Fitbit is one of the most consistent sleep trackers out there across all the studies. In the biggest independent sleep lab test (Schyvens 2025 - 62 healthy adults) Fitbit landed near the top for sleep staging for wrist based wearables, just behind Apple Watch. In a separate study of sleep clinic patients (Lee 2023 - 75 sleep clinic patients) it didn't drop accuracy like the others, scoring virtually the same in clinical vs healthy sleepers. Most trackers typically read better on healthy people and fall apart on clinical patients so that consistency is a real point in Fitbit's favor. All of these are independent studies, not funded by any device maker.

Summary across the studies:

• Total sleep time is a strength. Fitbit was off by only about 6 to 11 minutes on how long you slept, one of the smallest misses of any wrist device.
• The sleep stage split is weaker. Like every wearable Fitbit is only "moderate" at splitting light, deep, and REM stages. Best to treat your deep and REM breakdown as a guide, not an exact readout.
• Nobody nails wake. Fitbit like all other wearables tested here is not great at catching when you are lying awake in bed, so total sleep can read high.
• Consistency is impressive. Fitbit scored about 0.42 on the standard 0 to 1 accuracy scale and it held that same 0.42 with clinical patients. Apple for example fell from 0.53 in healthy sleepers to 0.30 in patients.
• The Google update looks like it should help. Google published its own 2026 research report showing their newer sleep algorithm scored more accurately against lab studies than the older Fitbit one (agreement rose from about 0.56 to 0.63). It's their own study though, not independent, so take it with a grain of salt.

Tried my best to keep this concise without dumping a ton of data into this and keep any personal biases out.

I also created a comparison page that lets you pick 1 or multiple brands to see which ones are stronger in different areas and goes into more detail than what I shared above. I do own this site but the tool is completely free to use and it links all sources used to create it: https://www.kygo.app/tools/sleep-tracker-accuracy

Here are all links to the sources:

• Schyvens 2025 - 62 healthy adults, 6 wrist devices
• Lee 2023 - 75 sleep clinic patients, 11 devices
• Herberger 2025 - 45 sleep clinic patients, 3 finger rings
• Guo 2025 - 98 sleep clinic patients, 1 wrist device
• Miller 2022 - 53 healthy adults, 6 devices
• Miller 2020 - 12 healthy adults, Whoop
• Searles 2026 - 32 adults (young and older), Fitbit and Oura
• Robbins 2024 - 35 healthy adults, 3 devices (Oura funded)
• Svensson 2024 - 96 healthy adults, Oura only (Oura linked)
• Dial 2025 - 13 adults, 5 devices, heart rate and HRV
• Khan 2025 - meta analysis, 6 studies / 388 people, Oura
• JCSM 2025 - meta analysis, 24 studies / 798 people, wrist devices
• Haghayegh 2019 - Fitbit meta analysis
• Gong 2025 - review, 107 studies / ~100,000 people, smart rings

The data and research centralization took a few weeks to sort out and had multiple people contribute/review it so I hope you enjoy and thank you to everyone who helped with this!

Wake ups throughout the night cost you a piece of the night's recovery. So here's a guide I built to better understand what influences staying asleep and focuses mostly on Wake After Sleep Onset. I broke this into 5 main areas: nutrition, supplements, exercise, environment, and demographics. There's a plain english explanation column for each row/short definitions to try to help make it easier to understand each one briefly. All sources are linked in the third column as I hope this guide makes it easier for you to do your own research into each factor further.

Thank you all for being so receptive to my last post! I have been posting these in other communities over the last year so I'll try to share one here once a week. I know tables aren't ideal on mobile, so I'll include a mobile friendly version of this resource in the comments

Acronyms:
WASO - Wake After Sleep Onset
PSG - Polysomnography
SMD - Standardized Mean Difference
RCT - Randomized Controlled Trial
SWS - Slow Wave Sleep
AHI - Apnea-Hypopnea Index
SWSD - Shift Work Sleep Disorder
HPA - Hypothalamic-Pituitary-Adrenal (axis)

Nutrition

Factor	Impact	Key Info (study/effect size)	Plain English	Evidence
Dietary Fiber	Decrease Arousals	St-Onge 2016 n=26, PSG, controlled crossover	More fiber = fewer nighttime wake-ups	Strong
Sugar / Refined Carbs	Increase Arousals	St-Onge 2016 n=26, PSG, significant predictor	Sugar directly increased sleep arousals	Strong
Caffeine	Increase WASO +12 min	Gardiner 2023 meta-analysis, 24 studies	Caffeine adds ~12 min of nighttime waking	Strong
Alcohol	Increase Fragmentation	Spadola 2019 Jackson Heart Study, n=785, actigraphy	Sleep breaks apart as alcohol metabolizes	Strong
Late Eating (<1hr)	Increase WASO 2–2.6× odds	Crispim 2022 British J Nutrition, large n	Eating right before bed doubles wake-ups	Moderate
Tart Cherry Juice	Decrease WASO ~17 min	Pigeon 2010 n=15, RCT crossover, insomnia cohort	Cherry juice cut nighttime waking vs placebo	Moderate

Supplements

Factor	Impact	Key Info (study/effect size)	Plain English	Evidence
Melatonin (immediate-release)	No significant WASO effect	Moon 2022 meta-analysis of RCTs, Neuropsychopharmacology	Standard melatonin does not help you stay asleep	Strong
Ashwagandha (600mg/day)	Decrease WASO, SMD −0.39	Cheah 2021 meta-analysis, 5 RCTs, n=400 (3 trials/281 for WASO)	Ashwagandha reduced nighttime waking	Strong
Glycine (3g)	Decrease WASO, faster SWS onset	Yamadera 2007 n=11, PSG-measured, crossover	Glycine reduced waking and deepened sleep	Moderate
Magnesium (500mg)	Increase Sleep efficiency (elderly)	Abbasi 2012 RCT, n=46, 8-week, 65+ years	Improved efficiency but no direct WASO data	Limited
L-Theanine (200–450mg)	Mixed WASO results	Cotter 2025 benefits at 200–450mg/day	Some maintenance benefit but inconsistent alone	Limited
Valerian Root	No consistent WASO benefit	Shinjyo 2020 meta-analysis, 60 studies, n=6894	Subjective improvement only, no objective WASO change	Weak

Exercise

Factor	Impact	Key Info (study/effect size)	Plain English	Evidence
Moderate Aerobic Exercise	Decrease WASO ~10 min	Riedel 2024 meta-analysis of RCTs, insomnia patients	Regular cardio cuts ~10 min of nighttime waking	Strong
Resistance Training	Decrease Sleep disturbance, Increase efficiency	Kovacevic 2018 systematic review, 13 studies, n=652	Strength training improved mid-sleep disturbance	Moderate
Yoga	Decrease WASO ~56 min	Bu 2025 network meta-analysis, 22 RCTs, n=1348	Yoga showed large WASO reduction in insomnia patients	Low
Evening Moderate Exercise	Decrease WASO	Dolezal 2017 systematic review, 34 studies	Moderate evening exercise helps you stay asleep	Moderate
Vigorous Exercise ≤1hr Before Bed	Increase WASO risk	Stutz 2019 meta-analysis, 23 studies, Sports Medicine	Intense exercise right before bed may fragment sleep	Moderate

Environment

Factor	Impact	Key Info (study/effect size)	Plain English	Evidence
Bedroom Temp (20–25°C)	Decrease WASO at optimal range	Akiyama 2021 PSG-measured, 20–25°C optimal	Too hot or cold increases nighttime waking	Strong
Light at Night (even dim)	Increase WASO	Cho 2016 n=23, PSG, 5–10 lux, Chronobiology Int	Even dim light during sleep increases wake time	Strong
Noise (>50 dBA)	Increase WASO +30 min	Basner 2018 WHO systematic review, 74 studies	Noise above 50 dB adds ~30 min of waking	Strong
CO2 >1000 ppm (poor ventilation)	Increase Wake time +5 min	Kang 2024 n=36, field-lab, 3 ventilation levels	Stuffy bedroom air measurably fragments sleep	Moderate
Mattress (medium-firm)	Decrease Most consistent WASO	Hu 2025 n=12, PSG, 3 firmness levels compared	Medium-firm mattress gave most stable sleep	Limited

Demographics

Factor	Impact	Key Info (study/effect size)	Plain English	Evidence
Aging (30–60+)	Increase WASO ~10 min/decade	Ohayon 2004 meta-analysis, 65 studies, 3,577 subjects	Each decade adds ~10 min of nighttime waking	Strong
Female Sex	Paradox: more complaints, better PSG	Ohayon 2004 women better objective metrics, more subjective complaints	Women report worse sleep but objectively sleep better	Strong
Menopause (hot flashes)	Increase WASO, 69% flashes  awakening	Joffe 2013 PSG + GnRH model, n=29, hot flashes = 27% of WASO	Nighttime hot flashes are a major wake trigger	Strong
Obesity (BMI ≥30)	Increase WASO significantly	Zhao 2021 Sleep Heart Health Study, n=5,723, PSG	Higher WASO independently associated with obesity	Strong
Shift Work	Increase WASO, Decrease sleep efficiency	Wickwire 2017 narrative review, SWSD patients, Chest	Shift workers have more fragmented daytime sleep	Moderate
Nocturia (≥2 episodes)	Increase WASO +34 min	Fung 2017 SOF study, n=1,520, actigraphy	More bathroom trips = much more nighttime waking	Strong
Obstructive Sleep Apnea	Increase WASO, Increase arousals with severity	Patel 2019 comprehensive review, PSG data	Each breathing event triggers arousal and waking	Strong
Chronic Pain	Increase WASO, large effect	Mathias 2018 meta-analysis, 37 studies, PSG	Pain significantly increases nighttime wake time	Strong
Psychological Stress	Increase WASO via cortisol elevation	Vgontzas 2001 n=24, 24-hr cortisol sampling, PSG	Stress hormones directly fragment sleep	Moderate

I pulled 14 sleep lab (polysomnography) studies across Fitbit, Oura, Apple Watch, Whoop and Garmin to try to identify what wearable has the "most accurate" sleep tracking across different sleep metrics. Pixel Watch utilizes the same sleep algorithm as Fitbit (Google Health) but has fewer independent studies using their hardware so this will blend studies between the two. Here's a summary of what I found.

Short version: Pixel Watch/Fitbit's algorithm is one of the most consistent across all the studies. In the biggest independent sleep lab test (Schyvens 2025 - 62 healthy adults) it landed near the top for sleep staging for wrist based wearables, just behind Apple Watch. In a separate study of sleep clinic patients (Lee 2023 - 75 sleep clinic patients) it didn't drop accuracy like the others, scoring virtually the same in clinical vs healthy sleepers. Most trackers typically read better on healthy people and fall apart on clinical patients so that consistency is a real point in it's favor. None of these studies are Google or Fitbit funded.

Summary across the studies:

• Pixel Watch and Fitbit scored almost identically in the one study that tested both. Pixel Watch scored 0.40, Fitbit 0.42, on the standard 0 to 1 accuracy scale. The authors group them in the same performance tier, and a gap that size is mostly noise.
• Total sleep time is a strength. Off by only about 6 to 11 minutes on how long you slept, one of the smallest misses of any wrist device.
• The sleep stage split is weaker. Like every wearable it's only "moderate" at splitting light, deep, and REM stages. Best to treat your deep and REM breakdown as a guide, not an exact readout.
• Nobody nails wake. Similar to all other wearables tested here it's not great at catching when you are lying awake in bed, so total sleep can read high.
• Consistency is impressive. It scored about 0.42 for healthy sleepers and it held at 0.42 with clinical patients. Apple for example fell from 0.53 in healthy sleepers to 0.30 in patients.
• The Google update looks like it should help. Google published its own (2026 research report) showing their newer sleep algorithm scored more accurately against lab studies than the older Fitbit one (agreement rose from about 0.56 to 0.63). It's their own study though, not independent, so take it with a grain of salt.

Tried my best to keep this helpful yet concise. I'll include the full comparison in the comments if you'd like to see how the other brands compared as well as all sources.

I get that there's varying opinions on the importance of an individual's absolute VO2 max number. Personally how VO2 max trends over time is what is more interesting to me as I find it a good signal to track long term health/longevity and how much overall fitness you're gaining or losing over time.

If this is a topic you're interested in learning more about I've built out this list of factors that influence it as a starting point for anyone to learn more and do their own research. To make this easier I've broken this down by category, included a short plain english definition, and linked all sources (scroll to the right on the tables and you'll see the hyperlinks). I've removed a few from here as it was getting far too long and sorry I know the tables are a bit of a pain on mobile.

Exercise

Factor	Impact	Study/effect	Plain English	Evidence rank
HIIT (long interval, high volume)	Increase	Wen 2019. Meta, 53 RCTs. SMD 0.50 to 2.48 vs control, 0.65 to 1.07 vs steady cardio	Intervals of 2 min+ are the only version that beats steady cardio	Strong
Detraining	Decrease	Zheng 2022. 21 athlete studies. Under 30 days ES −0.62 (−3.93%). Over 30 days ES −1.42 (−9.43%)	Under a month off costs about 4%, longer about 9%	Strong
Moderate continuous cardio	Increase	Wen 2019. Overall SMD 0.41 to 1.81 across populations	Steady cardio works, it is just less time efficient	Strong
Sprint interval training	Increase	Sloth 2013. 19 studies, 13 with VO2max data. g = 0.63 (0.39 to 0.87), +4.2 to 13.4%	Very short all out sprints reliably raise it	Moderate to Strong
Resistance training (alone)	Increase	Smart 2022. 37 studies, 22 pooled. +1.89 mL/kg/min (1.21 to 2.57) under 24 weeks. Null beyond 24 weeks	Small bump in over 60s, and only in shorter programs	Moderate

Nutrition & Supplements

Factor	Impact	Study/effect	Plain English	Evidence rank
Dietary nitrate / beetroot	No effect	Gao 2021. Meta, 73 studies, n=1,061. Submaximal VO2 −0.04 L/min (p<0.00001). VO2max not significant	Makes a given pace cheaper to hold. It does not raise your ceiling	Strong
Creatine	Slight decrease	Gras 2023. Meta, 19 RCTs, n=424. ES −0.32 (−0.51 to −0.12)	Adds body water, so the per kilo number dips slightly	Strong
Iron (when deficient)	Increase	Pasricha 2014. 24 RCTs, 18 for this figure. +2.35 mL/kg/min (0.82 to 3.88)	Fixing low iron raises it. This is correcting a deficit, not a boost	Moderate to Strong
Caffeine	No effect	Brietzke 2017. RCT, n=9. Time to exhaustion +18.7%, peak power +13%, VO2max unchanged	Helps you push harder. Much of the benefit is placebo	Moderate

Demographics

Factor	Impact	Study/effect	Plain English	Evidence rank
Body fat / excess weight	Decrease (per kg)	van Baak 2021. 66 RCTs, n=3,964. Training +3.82 mL/kg/min (3.17 to 4.48)	The score is per kilo, so losing fat raises it arithmetically	Strong
Age	Decrease	Fleg 2005. Baltimore Longitudinal Study, n=810. 3 to 6% per decade in the 20s and 30s, over 20% from the 70s	Decline accelerates with age, and faster in men from the 40s onward	Strong
Genetics (trainability)	Variable	Bouchard 1999. HERITAGE, n=481 across 98 families. Maximum heritability estimate 47%	Same training, wildly different response. It runs in families	Strong
Dehydration	Decrease	Cheuvront 2010. Mechanism review. Threshold is over 2% of body mass	Less blood volume to pump, and heat makes it worse	Moderate
Sex	Variable	Santisteban 2022. Review. 10 to 12% performance gap. Elite women about 10% lower per kg	Mostly oxygen delivery, meaning haemoglobin mass and heart size	Moderate
Immobilization / bed rest	Decrease	Saltin 1968 n=5, with McGavock 2009 follow up. −26% in three weeks	Three weeks flat on your back costs about what 40 years of aging does. Crazy.	Moderate

Lifestyle & Environment

Factor	Impact	Study/effect	Plain English	Evidence rank
Sedentary behaviour	Decrease	Eriksen 2016. n=16,025. No association among the highly active	Matters most if you are not already training hard	Moderate (observational)
Sauna bathing (habitual)	Increase	Kunutsor 2024. 2,012 men. +0.30 mL/kg/min per weekly session. Lee 2022 RCT +2.7 (0.2 to 5.3)	A small extra bump stacked on top of training	Moderate
Smoking	Decrease	Caci 2025. n=70, step test estimate. 38.8 vs 41.6 in never smokers	Lower aerobic ceiling. Whether quitting reverses it is untested here	Moderate
Altitude (live high, train low)	Increase	Wehrlin 2006. 10 elite orienteers at 2,500 m for 18 h/day over 24 days. VO2max 3,515 to 3,660 mL/min	Sleeping high builds more oxygen carrying red cells	Moderate
Heat acclimation	Increase	Waldron 2021. 28 articles. Hedges' g 0.42 cool, 0.63 hot. Lorenzo 2010 +5% cool, +8% hot	Plasma volume expands. Clearest when you are also tested in heat	Moderate

I was hoping to be able to create a more 1 to 1 comparison to identify which of the major wearable brands/devices had the most accurate heart rate accuracy but ultimately there wasn't a real clear cut answer on this. Instead I wanted to share some of the Apple specific findings that I found more useful to know.

Short summary:
Apple has more independent research for heart rate than any other brand and it's one of only two that publishes its own bad numbers. Recent generations have shown improvement both in independent and Apples own validation studies. Darker skin and arm movement both hurt accuracy. Placement of watch seems to be an easy way to improve readings.

Findings:

• Apple does publish its good and bad results openly unlike most brands.  Apple publishes its accuracy figures by activity including where it performs worse. Sedentary 98%, cycling 96%, yoga 92%, HIIT 91%, running 88%, walking 87%. The only other brand I saw that did this was Polar (Apple white paper 2024).
• Apple's sensor changes on series 6 or newer show actual improvement. Apple's own study showed series 6 or newer had 89% of readings land within 5 beats of real heart rate where series 4,5, and SE showed 72%. A separate analysis pooling 22 studies found the same jump, but in consistency. Older series ranged from 11 beats too low to 12 beats too high and newer dropped to 4 beats too low to 3 too high (Apple white paper 2024 & Lambe 2026).
• If you have darker skin its accuracy worsens. The group with the lightest skin in the study was off by under 1 beat on average. The group with the darkest skin was off by 8.4 beats and the range of readings for this group was 4.6 times wider. It was a smaller study but the results were pretty apparent (Chase 2024).
• Holding onto handles of elliptical roughly doubles the error . Hands free Apple Watch was off by 3.2% and read about half a beat low. Holding the arm levers it was off by 6.5% and read 5.5 beats low. Agreement dropped from 0.94 to 0.75 too. The arm movement problem shows up consistently across multiple studies/brands not just for ellipticals(Gillinov 2017).
• One easy thing on watch placement to help improve accuracy. Moving your watch from just above the wrist bone to three finger widths up the arm cut error by 11.4 percentage points and took agreement from 0.59 to 0.92. This was tested with a Fitbit rather than Apple Watch but the studies purpose was around sensor placement not brand (Vermunicht 2025).

I'll include summaries and links to the studies in the comment so anyone can check these out. If anyone has newer studies I missed please share and I'll update accordingly!

I pulled 14 sleep lab (polysomnography) studies across Apple Watch, Fitbit, Oura, Whoop and Garmin to try to identify what wearable has the "most accurate" sleep tracking across different sleep metrics. Here's a summary of what I found for Apple Watch.

Short version: Apple Watch is the most accurate sleep stage tracker among wrist wearables for healthy sleepers. In the independent sleep lab study (Schyvens 2025 - 62 healthy adults) it came out on top for splitting your night into light, deep and REM, and it was the best at catching REM specifically. It's main edge is only for healthy sleepers though and drops in people with disrupted sleep. Similar to all wrist based wearables it runs long on total sleep time. All of these are independent studies, not funded by Apple or showing any other biases, best I can tell.

Summary across the studies:

• Best at sleep staging. Apple scored about 0.53 on the 0 to 1 accuracy scale for splitting light, deep and REM. This was the highest score for wrist device and ahead of Fitbit, Whoop and Garmin (Schyvens 2025).
• Best at catching REM. Apple correctly identified about 69% of REM sleep, also the best of any device in that test (Schyvens 2025).
• It typically runs long on total sleep. Apple over counted how long individuals slept by about +20 minutes (Schyvens 2025). More than Oura did (-3 mins) but relatively normal range compared to the others.
• It struggles with disrupted sleep. Apple's staging dropped from 0.53 in healthy sleepers to 0.30 in sleep clinic patients (Lee 2023 - 75 sleep clinic patients). Bigger fall than Fitbit which held steady (0.42 vs 0.42).
• Nobody nails wake. Apple is the best of the group at catching when you are lying awake, but even it only spots about half those moments (Schyvens 2025). So total sleep reads a bit high across the board.
• Apple's own research puts its staging a bit higher. It came in at 0.63 in their own study but it's not independent so I went with the 0.53.

Tried my best to keep this concise without dumping a ton of data into this and keep any personal biases out. All 14 sources will be linked in the comments so you can check these out for yourself.

I understand there's varying opinions on the importance of an individual's absolute VO2 max number. Personally how VO2 max trends over time is what is more interesting to me as I find it a good signal to track long term health/longevity and how much overall fitness you're gaining or losing over time.

If this is a topic you're interested in learning more about I've built out this list of factors that influence it as a starting point for anyone to learn more and do their own research. To make this easier I've broken this down by category, included a short plain english definition, and linked all sources (scroll to the right on the tables and you'll see the hyperlinks). I've removed a few from here as it was getting far too long and sorry I know the tables aren't great on mobile but I'll include the full resource in the comments.

Exercise

Factor	Impact	Study/effect	Plain English	Evidence rank
HIIT (long interval, high volume)	Increase	Wen 2019. Meta, 53 RCTs. SMD 0.50 to 2.48 vs control, 0.65 to 1.07 vs steady cardio	Intervals of 2 min+ are the only version that beats steady cardio	Strong
Detraining	Decrease	Zheng 2022. 21 athlete studies. Under 30 days ES −0.62 (−3.93%). Over 30 days ES −1.42 (−9.43%)	Under a month off costs about 4%, longer about 9%	Strong
Moderate continuous cardio	Increase	Wen 2019. Overall SMD 0.41 to 1.81 across populations	Steady cardio works, it is just less time efficient	Strong
Sprint interval training	Increase	Sloth 2013. 19 studies, 13 with VO2max data. g = 0.63 (0.39 to 0.87), +4.2 to 13.4%	Very short all out sprints reliably raise it	Moderate to Strong
Resistance training (alone)	Increase	Smart 2022. 37 studies, 22 pooled. +1.89 mL/kg/min (1.21 to 2.57) under 24 weeks. Null beyond 24 weeks	Small bump in over 60s, and only in shorter programs	Moderate

Nutrition & Supplements

Factor	Impact	Study/effect	Plain English	Evidence rank
Dietary nitrate / beetroot	No effect	Gao 2021. Meta, 73 studies, n=1,061. Submaximal VO2 −0.04 L/min (p<0.00001). VO2max not significant	Makes a given pace cheaper to hold. It does not raise your ceiling	Strong
Creatine	Slight decrease	Gras 2023. Meta, 19 RCTs, n=424. ES −0.32 (−0.51 to −0.12)	Adds body water, so the per kilo number dips slightly	Strong
Iron (when deficient)	Increase	Pasricha 2014. 24 RCTs, 18 for this figure. +2.35 mL/kg/min (0.82 to 3.88)	Fixing low iron raises it. This is correcting a deficit, not a boost	Moderate to Strong
Caffeine	No effect	Brietzke 2017. RCT, n=9. Time to exhaustion +18.7%, peak power +13%, VO2max unchanged	Helps you push harder. Much of the benefit is placebo	Moderate

Demographics

Factor	Impact	Study/effect	Plain English	Evidence rank
Body fat / excess weight	Decrease (per kg)	van Baak 2021. 66 RCTs, n=3,964. Training +3.82 mL/kg/min (3.17 to 4.48)	The score is per kilo, so losing fat raises it arithmetically	Strong
Age	Decrease	Fleg 2005. Baltimore Longitudinal Study, n=810. 3 to 6% per decade in the 20s and 30s, over 20% from the 70s	Decline accelerates with age, and faster in men from the 40s onward	Strong
Genetics (trainability)	Variable	Bouchard 1999. HERITAGE, n=481 across 98 families. Maximum heritability estimate 47%	Same training, wildly different response. It runs in families	Strong
Dehydration	Decrease	Cheuvront 2010. Mechanism review. Threshold is over 2% of body mass	Less blood volume to pump, and heat makes it worse	Moderate
Sex	Variable	Santisteban 2022. Review. 10 to 12% performance gap. Elite women about 10% lower per kg	Mostly oxygen delivery, meaning haemoglobin mass and heart size	Moderate
Immobilization / bed rest	Decrease	Saltin 1968 n=5, with McGavock 2009 follow up. −26% in three weeks	Three weeks flat on your back costs about what 40 years of aging does. Crazy.	Moderate

Lifestyle & Environment

Factor	Impact	Study/effect	Plain English	Evidence rank
Sedentary behaviour	Decrease	Eriksen 2016. n=16,025. No association among the highly active	Matters most if you are not already training hard	Moderate (observational)
Sauna bathing (habitual)	Increase	Kunutsor 2024. 2,012 men. +0.30 mL/kg/min per weekly session. Lee 2022 RCT +2.7 (0.2 to 5.3)	A small extra bump stacked on top of training	Moderate
Smoking	Decrease	Caci 2025. n=70, step test estimate. 38.8 vs 41.6 in never smokers	Lower aerobic ceiling. Whether quitting reverses it is untested here	Moderate
Altitude (live high, train low)	Increase	Wehrlin 2006. 10 elite orienteers at 2,500 m for 18 h/day over 24 days. VO2max 3,515 to 3,660 mL/min	Sleeping high builds more oxygen carrying red cells	Moderate
Heat acclimation	Increase	Waldron 2021. 28 articles. Hedges' g 0.42 cool, 0.63 hot. Lorenzo 2010 +5% cool, +8% hot	Plasma volume expands. Clearest when you are also tested in heat	Moderate

Not the typical watch face post but hope you find this helpful.

I've used my Fitbit air for a bit and started questioning the calorie burn so I pulled what research has been done on fitbit's calorie accuracy. Fitbit is the most studied wearable out there for calories burn although there's nothing on the the new Air specifically yet. Heres what I did find:

• On average it's mostly spot on - across the whole dataset the mean bias was about 0.19 kcal per minute which is basically zero.
• Averages hide a lot though - individual readings swung from roughly 5 kcal/min under to 6 kcal/min over so "accurate on average" and "accurate for your session" are not the same thing.
• Running is its strong suit - Fitbit landed within about 4 to 15% on running calories in the activity breakdowns.
• Walking is the weak spot (surprising on this) - the same studies had it overcounting walking calories by around 54%. At low intensity the heart rate signal is soft and the algorithm overshoots.
• It runs mostly off heart rate not steps - steps only matter because they raise your HR. Easy walking barely moves HR so the estimate wobbles while a run tracks effort cleanly so it does well.
• The Google Health switch does matter here - the old Food Plans calorie deficit goal that auto adjusted a weight loss target for you is retired. Active calories you burn are still folded into your daily under/over target though so that part did not go away. You can still set a manual target too.
• Newer gear is still unproven - there is no independent peer reviewed calorie validation for the Charge 6, Sense 2, or the new Fitbit Air yet best I can tell.

For me personally it has been alright for daily trends but tracked intake and a food scale will serve you far better than any wearable estimate. I understand that individual results will vary from what the studies showed but hope this is just in general helpful.

This is the study I pulled a lot of this info from fyi  (Chevance et al., JMIR mHealth 2022, 52 studies, 203 device comparisons) it's the most up to date peer reviewed study I could find but the models are older.

I understand there's varying opinions on the importance of an individual's absolute VO2 max number. Personally how VO2 max trends over time is what is more interesting to me as I find it a good signal to track long term health/longevity and how much overall fitness you're gaining or losing over time.

If this is a topic you're interested in learning more about I've built out this list of factors that influence it as a starting point for anyone to learn more and do their own research. To make this easier I've broken this down by category, included a short plain english definition, and linked all sources (scroll to the right on the tables and you'll see the hyperlinks). Sorry I know the tables are a pain for mobile readers. To make it easier I also made it into a page that's better suited for it which I'll include in the comments and at the bottom of the post

Exercise

Factor	Impact	Study/effect	Plain English	Evidence rank
HIIT (long interval, high volume)	Increase	Wen 2019. Meta, 53 RCTs. SMD 0.50 to 2.48 vs control, 0.65 to 1.07 vs steady cardio	Intervals of 2 min+ are the only version that beats steady cardio	Strong
Detraining	Decrease	Zheng 2022. 21 athlete studies. Under 30 days ES −0.62 (−3.93%). Over 30 days ES −1.42 (−9.43%)	Under a month off costs about 4%, longer about 9%	Strong
Moderate continuous cardio	Increase	Wen 2019. Overall SMD 0.41 to 1.81 across populations	Steady cardio works, it is just less time efficient	Strong
Sprint interval training	Increase	Sloth 2013. 19 studies, 13 with VO2max data. g = 0.63 (0.39 to 0.87), +4.2 to 13.4%	Very short all out sprints reliably raise it	Moderate to Strong
Resistance training (alone)	Increase	Smart 2022. 37 studies, 22 pooled. +1.89 mL/kg/min (1.21 to 2.57) under 24 weeks. Null beyond 24 weeks	Small bump in over 60s, and only in shorter programs	Moderate

Nutrition & Supplements

Factor	Impact	Study/effect	Plain English	Evidence rank
Dietary nitrate / beetroot	No effect	Gao 2021. Meta, 73 studies, n=1,061. Submaximal VO2 −0.04 L/min (p<0.00001). VO2max not significant	Makes a given pace cheaper to hold. It does not raise your ceiling	Strong
Creatine	Slight decrease	Gras 2023. Meta, 19 RCTs, n=424. ES −0.32 (−0.51 to −0.12)	Adds body water, so the per kilo number dips slightly	Strong
Iron (when deficient)	Increase	Pasricha 2014. 24 RCTs, 18 for this figure. +2.35 mL/kg/min (0.82 to 3.88)	Fixing low iron raises it. This is correcting a deficit, not a boost	Moderate to Strong
Caffeine	No effect	Brietzke 2017. RCT, n=9. Time to exhaustion +18.7%, peak power +13%, VO2max unchanged	Helps you push harder. Much of the benefit is placebo	Moderate

Demographics

Factor	Impact	Study/effect	Plain English	Evidence rank
Body fat / excess weight	Decrease (per kg)	van Baak 2021. 66 RCTs, n=3,964. Training +3.82 mL/kg/min (3.17 to 4.48)	The score is per kilo, so losing fat raises it arithmetically	Strong
Age	Decrease	Fleg 2005. Baltimore Longitudinal Study, n=810. 3 to 6% per decade in the 20s and 30s, over 20% from the 70s	Decline accelerates with age, and faster in men from the 40s onward	Strong
Genetics (trainability)	Variable	Bouchard 1999. HERITAGE, n=481 across 98 families. Maximum heritability estimate 47%	Same training, wildly different response. It runs in families	Strong
Dehydration	Decrease	Cheuvront 2010. Mechanism review. Threshold is over 2% of body mass	Less blood volume to pump, and heat makes it worse	Moderate
Sex	Variable	Santisteban 2022. Review. 10 to 12% performance gap. Elite women about 10% lower per kg	Mostly oxygen delivery, meaning haemoglobin mass and heart size	Moderate
Immobilization / bed rest	Decrease	Saltin 1968 n=5, with McGavock 2009 follow up. −26% in three weeks	Three weeks flat on your back costs about what 40 years of aging does. Crazy.	Moderate

Lifestyle & Environment

Factor	Impact	Study/effect	Plain English	Evidence rank
Sedentary behaviour	Decrease	Eriksen 2016. n=16,025. No association among the highly active	Matters most if you are not already training hard	Moderate (observational)
Sauna bathing (habitual)	Increase	Kunutsor 2024. 2,012 men. +0.30 mL/kg/min per weekly session. Lee 2022 RCT +2.7 (0.2 to 5.3)	A small extra bump stacked on top of training	Moderate
Smoking	Decrease	Caci 2025. n=70, step test estimate. 38.8 vs 41.6 in never smokers	Lower aerobic ceiling. Whether quitting reverses it is untested here	Moderate
Altitude (live high, train low)	Increase	Wehrlin 2006. 10 elite orienteers at 2,500 m for 18 h/day over 24 days. VO2max 3,515 to 3,660 mL/min	Sleeping high builds more oxygen carrying red cells	Moderate
Heat acclimation	Increase	Waldron 2021. 28 articles. Hedges' g 0.42 cool, 0.63 hot. Lorenzo 2010 +5% cool, +8% hot	Plasma volume expands. Clearest when you are also tested in heat	Moderate

Here's the more mobile friendly page I made with the data: https://www.kygo.app/tools/vo2-max-factors 
It's completely free to use, there's no paywall and I don't run ads. Just wanted to let you know what your clicking on upfront.

I understand there's varying opinions on the importance of an individual's absolute VO2 max number. Personally how VO2 max trends over time is what is more interesting to me as I find it a good signal to track long term health/longevity and how much overall fitness you're gaining or losing over time.

If this is a topic you're interested in learning more about I've built out this list of factors that influence it as a starting point for anyone to learn more and do their own research. To make this easier I've broken this down by category, included a short plain english definition, and linked all sources (scroll to the right on the tables and you'll see the hyperlinks). I've removed a few from here as it was getting far too long and sorry I know the tables aren't great on mobile but I'll include the full resource in the comments.

Exercise

Factor	Impact	Study/effect	Plain English	Evidence rank
HIIT (long interval, high volume)	Increase	Wen 2019. Meta, 53 RCTs. SMD 0.50 to 2.48 vs control, 0.65 to 1.07 vs steady cardio	Intervals of 2 min+ are the only version that beats steady cardio	Strong
Detraining	Decrease	Zheng 2022. 21 athlete studies. Under 30 days ES −0.62 (−3.93%). Over 30 days ES −1.42 (−9.43%)	Under a month off costs about 4%, longer about 9%	Strong
Moderate continuous cardio	Increase	Wen 2019. Overall SMD 0.41 to 1.81 across populations	Steady cardio works, it is just less time efficient	Strong
Sprint interval training	Increase	Sloth 2013. 19 studies, 13 with VO2max data. g = 0.63 (0.39 to 0.87), +4.2 to 13.4%	Very short all out sprints reliably raise it	Moderate to Strong
Resistance training (alone)	Increase	Smart 2022. 37 studies, 22 pooled. +1.89 mL/kg/min (1.21 to 2.57) under 24 weeks. Null beyond 24 weeks	Small bump in over 60s, and only in shorter programs	Moderate

Nutrition & Supplements

Factor	Impact	Study/effect	Plain English	Evidence rank
Dietary nitrate / beetroot	No effect	Gao 2021. Meta, 73 studies, n=1,061. Submaximal VO2 −0.04 L/min (p<0.00001). VO2max not significant	Makes a given pace cheaper to hold. It does not raise your ceiling	Strong
Creatine	Slight decrease	Gras 2023. Meta, 19 RCTs, n=424. ES −0.32 (−0.51 to −0.12)	Adds body water, so the per kilo number dips slightly	Strong
Iron (when deficient)	Increase	Pasricha 2014. 24 RCTs, 18 for this figure. +2.35 mL/kg/min (0.82 to 3.88)	Fixing low iron raises it. This is correcting a deficit, not a boost	Moderate to Strong
Caffeine	No effect	Brietzke 2017. RCT, n=9. Time to exhaustion +18.7%, peak power +13%, VO2max unchanged	Helps you push harder. Much of the benefit is placebo	Moderate

Demographics

Factor	Impact	Study/effect	Plain English	Evidence rank
Age	Decrease	Fleg 2005. Baltimore Longitudinal Study, n=810. 3 to 6% per decade in the 20s and 30s, over 20% from the 70s	Decline accelerates with age, and faster in men from the 40s onward	Strong
Genetics (trainability)	Variable	Bouchard 1999. HERITAGE, n=481 across 98 families. Maximum heritability estimate 47%	Same training, wildly different response. It runs in families	Strong
Dehydration	Decrease	Cheuvront 2010. Mechanism review. Threshold is over 2% of body mass	Less blood volume to pump, and heat makes it worse	Moderate
Sex	Variable	Santisteban 2022. Review. 10 to 12% performance gap. Elite women about 10% lower per kg	Mostly oxygen delivery, meaning haemoglobin mass and heart size	Moderate
Immobilization / bed rest	Decrease	Saltin 1968 n=5, with McGavock 2009 follow up. −26% in three weeks	Three weeks flat on your back costs about what 40 years of aging does. Crazy.	Moderate

Lifestyle & Environment

Factor	Impact	Study/effect	Plain English	Evidence rank
Sedentary behaviour	Decrease	Eriksen 2016. n=16,025. No association among the highly active	Matters most if you are not already training hard	Moderate (observational)
Sauna bathing (habitual)	Increase	Kunutsor 2024. 2,012 men. +0.30 mL/kg/min per weekly session. Lee 2022 RCT +2.7 (0.2 to 5.3)	A small extra bump stacked on top of training	Moderate
Smoking	Decrease	Caci 2025. n=70, step test estimate. 38.8 vs 41.6 in never smokers	Lower aerobic ceiling. Whether quitting reverses it is untested here	Moderate
Altitude (live high, train low)	Increase	Wehrlin 2006. 10 elite orienteers at 2,500 m for 18 h/day over 24 days. VO2max 3,515 to 3,660 mL/min	Sleeping high builds more oxygen carrying red cells	Moderate
Heat acclimation	Increase	Waldron 2021. 28 articles. Hedges' g 0.42 cool, 0.63 hot. Lorenzo 2010 +5% cool, +8% hot	Plasma volume expands. Clearest when you are also tested in heat	Moderate

I understand there's varying opinions on the importance of an individual's absolute VO2 max number. Personally how VO2 max trends over time is what is more interesting to me as I find it a good signal to track long term health/longevity and how much overall fitness you're gaining or losing over time.

I've built out this list of factors that influence it as a starting point for anyone to learn more and do their own research (this is my attempt). To make this easier I've broken this down by category, included a short plain english definition, and linked all sources (scroll to the right on the tables and you'll see the hyperlinks). I've removed a few from here as it was getting far too long and sorry I know the tables aren't great on mobile but I'll include the full resource in the comments.

Exercise

Factor	Impact	Study/effect	Plain English	Evidence rank
HIIT (long interval, high volume)	Increase	Wen 2019. Meta, 53 RCTs. SMD 0.50 to 2.48 vs control, 0.65 to 1.07 vs steady cardio	Intervals of 2 min+ are the only version that beats steady cardio	Strong
Detraining	Decrease	Zheng 2022. 21 athlete studies. Under 30 days ES −0.62 (−3.93%). Over 30 days ES −1.42 (−9.43%)	Under a month off costs about 4%, longer about 9%	Strong
Moderate continuous cardio	Increase	Wen 2019. Overall SMD 0.41 to 1.81 across populations	Steady cardio works, it is just less time efficient	Strong
Sprint interval training	Increase	Sloth 2013. 19 studies, 13 with VO2max data. g = 0.63 (0.39 to 0.87), +4.2 to 13.4%	Very short all out sprints reliably raise it	Moderate to Strong
Resistance training (alone)	Increase	Smart 2022. 37 studies, 22 pooled. +1.89 mL/kg/min (1.21 to 2.57) under 24 weeks. Null beyond 24 weeks	Small bump in over 60s, and only in shorter programs	Moderate

Nutrition & Supplements

Factor	Impact	Study/effect	Plain English	Evidence rank
Dietary nitrate / beetroot	No effect	Gao 2021. Meta, 73 studies, n=1,061. Submaximal VO2 −0.04 L/min (p<0.00001). VO2max not significant	Makes a given pace cheaper to hold. It does not raise your ceiling	Strong
Creatine	Slight decrease	Gras 2023. Meta, 19 RCTs, n=424. ES −0.32 (−0.51 to −0.12)	Adds body water, so the per kilo number dips slightly	Strong
Iron (when deficient)	Increase	Pasricha 2014. 24 RCTs, 18 for this figure. +2.35 mL/kg/min (0.82 to 3.88)	Fixing low iron raises it. This is correcting a deficit, not a boost	Moderate to Strong
Caffeine	No effect	Brietzke 2017. RCT, n=9. Time to exhaustion +18.7%, peak power +13%, VO2max unchanged	Helps you push harder. Much of the benefit is placebo	Moderate

Demographics

Factor	Impact	Study/effect	Plain English	Evidence rank
Age	Decrease	Fleg 2005. Baltimore Longitudinal Study, n=810. 3 to 6% per decade in the 20s and 30s, over 20% from the 70s	Decline accelerates with age, and faster in men from the 40s onward	Strong
Genetics (trainability)	Variable	Bouchard 1999. HERITAGE, n=481 across 98 families. Maximum heritability estimate 47%	Same training, wildly different response. It runs in families	Strong
Dehydration	Decrease	Cheuvront 2010. Mechanism review. Threshold is over 2% of body mass	Less blood volume to pump, and heat makes it worse	Moderate
Sex	Variable	Santisteban 2022. Review. 10 to 12% performance gap. Elite women about 10% lower per kg	Mostly oxygen delivery, meaning haemoglobin mass and heart size	Moderate
Immobilization / bed rest	Decrease	Saltin 1968 n=5, with McGavock 2009 follow up. −26% in three weeks	Three weeks flat on your back costs about what 40 years of aging does. Crazy.	Moderate

Lifestyle & Environment

Factor	Impact	Study/effect	Plain English	Evidence rank
Sedentary behaviour	Decrease	Eriksen 2016. n=16,025. No association among the highly active	Matters most if you are not already training hard	Moderate (observational)
Sauna bathing (habitual)	Increase	Kunutsor 2024. 2,012 men. +0.30 mL/kg/min per weekly session. Lee 2022 RCT +2.7 (0.2 to 5.3)	A small extra bump stacked on top of training	Moderate
Smoking	Decrease	Caci 2025. n=70, step test estimate. 38.8 vs 41.6 in never smokers	Lower aerobic ceiling. Whether quitting reverses it is untested here	Moderate
Altitude (live high, train low)	Increase	Wehrlin 2006. 10 elite orienteers at 2,500 m for 18 h/day over 24 days. VO2max 3,515 to 3,660 mL/min	Sleeping high builds more oxygen carrying red cells	Moderate
Heat acclimation	Increase	Waldron 2021. 28 articles. Hedges' g 0.42 cool, 0.63 hot. Lorenzo 2010 +5% cool, +8% hot	Plasma volume expands. Clearest when you are also tested in heat	Moderate

I understand there's many different opinions on the importance of an individual's absolute VO2 max number. Personally how VO2 max trends over time is what is more interesting to me as I find it a good signal to track long term health and how much overall fitness you're gaining or losing over time.

If this is a topic you're interested in learning more about I've built out this list of factors that influence it as a starting point for anyone to learn more and do their own research. To make this easier I've broken this down by category, included a short plain english definition, and linked all sources (scroll to the right on the tables and you'll see the hyperlinks). I've removed a few from here as it was getting far too long and sorry I know the tables aren't great on mobile but I'll include the full resource in the comments.

Exercise

Factor	Impact	Study/effect	Plain English	Evidence rank
HIIT (long interval, high volume)	Increase	Wen 2019. Meta, 53 RCTs. SMD 0.50 to 2.48 vs control, 0.65 to 1.07 vs steady cardio	Intervals of 2 min+ are the only version that beats steady cardio	Strong
Detraining	Decrease	Zheng 2022. 21 athlete studies. Under 30 days ES −0.62 (−3.93%). Over 30 days ES −1.42 (−9.43%)	Under a month off costs about 4%, longer about 9%	Strong
Moderate continuous cardio	Increase	Wen 2019. Overall SMD 0.41 to 1.81 across populations	Steady cardio works, it is just less time efficient	Strong
Sprint interval training	Increase	Sloth 2013. 19 studies, 13 with VO2max data. g = 0.63 (0.39 to 0.87), +4.2 to 13.4%	Very short all out sprints reliably raise it	Moderate to Strong
Resistance training (alone)	Increase	Smart 2022. 37 studies, 22 pooled. +1.89 mL/kg/min (1.21 to 2.57) under 24 weeks. Null beyond 24 weeks	Small bump in over 60s, and only in shorter programs	Moderate

Nutrition & Supplements

Factor	Impact	Study/effect	Plain English	Evidence rank
Dietary nitrate / beetroot	No effect	Gao 2021. Meta, 73 studies, n=1,061. Submaximal VO2 −0.04 L/min (p<0.00001). VO2max not significant	Makes a given pace cheaper to hold. It does not raise your ceiling	Strong
Creatine	Slight decrease	Gras 2023. Meta, 19 RCTs, n=424. ES −0.32 (−0.51 to −0.12)	Adds body water, so the per kilo number dips slightly	Strong
Iron (when deficient)	Increase	Pasricha 2014. 24 RCTs, 18 for this figure. +2.35 mL/kg/min (0.82 to 3.88)	Fixing low iron raises it. This is correcting a deficit, not a boost	Moderate to Strong
Caffeine	No effect	Brietzke 2017. RCT, n=9. Time to exhaustion +18.7%, peak power +13%, VO2max unchanged	Helps you push harder. Much of the benefit is placebo	Moderate

Demographics

Factor	Impact	Study/effect	Plain English	Evidence rank
Age	Decrease	Fleg 2005. Baltimore Longitudinal Study, n=810. 3 to 6% per decade in the 20s and 30s, over 20% from the 70s	Decline accelerates with age, and faster in men from the 40s onward	Strong
Genetics (trainability)	Variable	Bouchard 1999. HERITAGE, n=481 across 98 families. Maximum heritability estimate 47%	Same training, wildly different response. It runs in families	Strong
Dehydration	Decrease	Cheuvront 2010. Mechanism review. Threshold is over 2% of body mass	Less blood volume to pump, and heat makes it worse	Moderate
Sex	Variable	Santisteban 2022. Review. 10 to 12% performance gap. Elite women about 10% lower per kg	Mostly oxygen delivery, meaning haemoglobin mass and heart size	Moderate
Immobilization / bed rest	Decrease	Saltin 1968 n=5, with McGavock 2009 follow up. −26% in three weeks	Three weeks flat on your back costs about what 40 years of aging does. Crazy.	Moderate

Lifestyle & Environment

Factor	Impact	Study/effect	Plain English	Evidence rank
Sedentary behaviour	Decrease	Eriksen 2016. n=16,025. No association among the highly active	Matters most if you are not already training hard	Moderate (observational)
Sauna bathing (habitual)	Increase	Kunutsor 2024. 2,012 men. +0.30 mL/kg/min per weekly session. Lee 2022 RCT +2.7 (0.2 to 5.3)	A small extra bump stacked on top of training	Moderate
Smoking	Decrease	Caci 2025. n=70, step test estimate. 38.8 vs 41.6 in never smokers	Lower aerobic ceiling. Whether quitting reverses it is untested here	Moderate
Altitude (live high, train low)	Increase	Wehrlin 2006. 10 elite orienteers at 2,500 m for 18 h/day over 24 days. VO2max 3,515 to 3,660 mL/min	Sleeping high builds more oxygen carrying red cells	Moderate
Heat acclimation	Increase	Waldron 2021. 28 articles. Hedges' g 0.42 cool, 0.63 hot. Lorenzo 2010 +5% cool, +8% hot	Plasma volume expands. Clearest when you are also tested in heat	Moderate

I understand there's varying opinions on the importance of an individual's absolute VO2 max number. Personally how VO2 max trends over time is what is more interesting to me as I find it a good signal to track long term health/longevity and how much overall fitness you're gaining or losing over time.

If this is a topic you're interested in learning more about I've built out this list of factors that influence it as a starting point for anyone to learn more and do their own research. To make this easier I've broken this down by category, included a short plain english definition, and linked all sources (scroll to the right on the tables and you'll see the hyperlinks). I've removed a few from here as it was getting far too long and sorry I know the tables aren't great on mobile but I'll include the full resource in the comments.

Exercise

Factor	Impact	Study/effect	Plain English	Evidence rank
HIIT (long interval, high volume)	Increase	Wen 2019. Meta, 53 RCTs. SMD 0.50 to 2.48 vs control, 0.65 to 1.07 vs steady cardio	Intervals of 2 min+ are the only version that beats steady cardio	Strong
Detraining	Decrease	Zheng 2022. 21 athlete studies. Under 30 days ES −0.62 (−3.93%). Over 30 days ES −1.42 (−9.43%)	Under a month off costs about 4%, longer about 9%	Strong
Moderate continuous cardio	Increase	Wen 2019. Overall SMD 0.41 to 1.81 across populations	Steady cardio works, it is just less time efficient	Strong
Sprint interval training	Increase	Sloth 2013. 19 studies, 13 with VO2max data. g = 0.63 (0.39 to 0.87), +4.2 to 13.4%	Very short all out sprints reliably raise it	Moderate to Strong
Resistance training (alone)	Increase	Smart 2022. 37 studies, 22 pooled. +1.89 mL/kg/min (1.21 to 2.57) under 24 weeks. Null beyond 24 weeks	Small bump in over 60s, and only in shorter programs	Moderate

Nutrition & Supplements

Factor	Impact	Study/effect	Plain English	Evidence rank
Dietary nitrate / beetroot	No effect	Gao 2021. Meta, 73 studies, n=1,061. Submaximal VO2 −0.04 L/min (p<0.00001). VO2max not significant	Makes a given pace cheaper to hold. It does not raise your ceiling	Strong
Creatine	Slight decrease	Gras 2023. Meta, 19 RCTs, n=424. ES −0.32 (−0.51 to −0.12)	Adds body water, so the per kilo number dips slightly	Strong
Iron (when deficient)	Increase	Pasricha 2014. 24 RCTs, 18 for this figure. +2.35 mL/kg/min (0.82 to 3.88)	Fixing low iron raises it. This is correcting a deficit, not a boost	Moderate to Strong
Caffeine	No effect	Brietzke 2017. RCT, n=9. Time to exhaustion +18.7%, peak power +13%, VO2max unchanged	Helps you push harder. Much of the benefit is placebo	Moderate

Demographics

Factor	Impact	Study/effect	Plain English	Evidence rank
Age	Decrease	Fleg 2005. Baltimore Longitudinal Study, n=810. 3 to 6% per decade in the 20s and 30s, over 20% from the 70s	Decline accelerates with age, and faster in men from the 40s onward	Strong
Genetics (trainability)	Variable	Bouchard 1999. HERITAGE, n=481 across 98 families. Maximum heritability estimate 47%	Same training, wildly different response. It runs in families	Strong
Dehydration	Decrease	Cheuvront 2010. Mechanism review. Threshold is over 2% of body mass	Less blood volume to pump, and heat makes it worse	Moderate
Sex	Variable	Santisteban 2022. Review. 10 to 12% performance gap. Elite women about 10% lower per kg	Mostly oxygen delivery, meaning haemoglobin mass and heart size	Moderate
Immobilization / bed rest	Decrease	Saltin 1968 n=5, with McGavock 2009 follow up. −26% in three weeks	Three weeks flat on your back costs about what 40 years of aging does. Crazy.	Moderate

Lifestyle & Environment

Factor	Impact	Study/effect	Plain English	Evidence rank
Sedentary behaviour	Decrease	Eriksen 2016. n=16,025. No association among the highly active	Matters most if you are not already training hard	Moderate (observational)
Sauna bathing (habitual)	Increase	Kunutsor 2024. 2,012 men. +0.30 mL/kg/min per weekly session. Lee 2022 RCT +2.7 (0.2 to 5.3)	A small extra bump stacked on top of training	Moderate
Smoking	Decrease	Caci 2025. n=70, step test estimate. 38.8 vs 41.6 in never smokers	Lower aerobic ceiling. Whether quitting reverses it is untested here	Moderate
Altitude (live high, train low)	Increase	Wehrlin 2006. 10 elite orienteers at 2,500 m for 18 h/day over 24 days. VO2max 3,515 to 3,660 mL/min	Sleeping high builds more oxygen carrying red cells	Moderate
Heat acclimation	Increase	Waldron 2021. 28 articles. Hedges' g 0.42 cool, 0.63 hot. Lorenzo 2010 +5% cool, +8% hot	Plasma volume expands. Clearest when you are also tested in heat	Moderate

I understand there's varying opinions on the importance of an individual's absolute VO2 max number. Personally how VO2 max trends over time is what is more interesting to me as I find it a good signal to track long term health and how much overall fitness you're gaining or losing over time.

If this is a topic you're interested in learning more about I've built out this list of factors that influence it as a starting point for anyone to learn more and do their own research. To make this easier I've broken this down by category, included a short plain english definition, and linked all sources. I've removed a few from here as it was getting far too long but I'll include the full resource in the comments.

Exercise

Factor	Impact	Study/effect	Plain English	Evidence rank
HIIT (long interval, high volume)	Increase	Wen 2019. Meta, 53 RCTs. SMD 0.50 to 2.48 vs control, 0.65 to 1.07 vs steady cardio	Intervals of 2 min+ are the only version that beats steady cardio	Strong
Detraining	Decrease	Zheng 2022. 21 athlete studies. Under 30 days ES −0.62 (−3.93%). Over 30 days ES −1.42 (−9.43%)	Under a month off costs about 4%, longer about 9%	Strong
Moderate continuous cardio	Increase	Wen 2019. Overall SMD 0.41 to 1.81 across populations	Steady cardio works, it is just less time efficient	Strong
Sprint interval training	Increase	Sloth 2013. 19 studies, 13 with VO2max data. g = 0.63 (0.39 to 0.87), +4.2 to 13.4%	Very short all out sprints reliably raise it	Moderate to Strong
Resistance training (alone)	Increase	Smart 2022. 37 studies, 22 pooled. +1.89 mL/kg/min (1.21 to 2.57) under 24 weeks. Null beyond 24 weeks	Small bump in over 60s, and only in shorter programs	Moderate

Nutrition & Supplements

Factor	Impact	Study/effect	Plain English	Evidence rank
Dietary nitrate / beetroot	No effect	Gao 2021. Meta, 73 studies, n=1,061. Submaximal VO2 −0.04 L/min (p<0.00001). VO2max not significant	Makes a given pace cheaper to hold. It does not raise your ceiling	Strong
Creatine	Slight decrease	Gras 2023. Meta, 19 RCTs, n=424. ES −0.32 (−0.51 to −0.12)	Adds body water, so the per kilo number dips slightly	Strong
Iron (when deficient)	Increase	Pasricha 2014. 24 RCTs, 18 for this figure. +2.35 mL/kg/min (0.82 to 3.88)	Fixing low iron raises it. This is correcting a deficit, not a boost	Moderate to Strong
Caffeine	No effect	Brietzke 2017. RCT, n=9. Time to exhaustion +18.7%, peak power +13%, VO2max unchanged	Helps you push harder. Much of the benefit is placebo	Moderate

Demographics

Factor	Impact	Study/effect	Plain English	Evidence rank
Age	Decrease	Fleg 2005. Baltimore Longitudinal Study, n=810. 3 to 6% per decade in the 20s and 30s, over 20% from the 70s	Decline accelerates with age, and faster in men from the 40s onward	Strong
Genetics (trainability)	Variable	Bouchard 1999. HERITAGE, n=481 across 98 families. Maximum heritability estimate 47%	Same training, wildly different response. It runs in families	Strong
Dehydration	Decrease	Cheuvront 2010. Mechanism review. Threshold is over 2% of body mass	Less blood volume to pump, and heat makes it worse	Moderate
Sex	Variable	Santisteban 2022. Review. 10 to 12% performance gap. Elite women about 10% lower per kg	Mostly oxygen delivery, meaning haemoglobin mass and heart size	Moderate
Immobilization / bed rest	Decrease	Saltin 1968 n=5, with McGavock 2009 follow up. −26% in three weeks	Three weeks flat on your back costs about what 40 years of aging does. Crazy.	Moderate

Lifestyle & Environment

Factor	Impact	Study/effect	Plain English	Evidence rank
Sedentary behaviour	Decrease	Eriksen 2016. n=16,025. No association among the highly active	Matters most if you are not already training hard	Moderate (observational)
Sauna bathing (habitual)	Increase	Kunutsor 2024. 2,012 men. +0.30 mL/kg/min per weekly session. Lee 2022 RCT +2.7 (0.2 to 5.3)	A small extra bump stacked on top of training	Moderate
Smoking	Decrease	Caci 2025. n=70, step test estimate. 38.8 vs 41.6 in never smokers	Lower aerobic ceiling. Whether quitting reverses it is untested here	Moderate
Altitude (live high, train low)	Increase	Wehrlin 2006. 10 elite orienteers at 2,500 m for 18 h/day over 24 days. VO2max 3,515 to 3,660 mL/min	Sleeping high builds more oxygen carrying red cells	Moderate
Heat acclimation	Increase	Waldron 2021. 28 articles. Hedges' g 0.42 cool, 0.63 hot. Lorenzo 2010 +5% cool, +8% hot	Plasma volume expands. Clearest when you are also tested in heat	Moderate