u/DriftNapper

As a clinical sleep researcher, one of the most humbling findings in the field is this: humans are genuinely bad at sensing their own sleep. Not in a trivial way, in a fundamental, measurable, clinically meaningful way. We don't have a proprioceptive sense for sleep the way we do for hunger, thirst, balance, or fatigue. Sleep happens to us, and most of what matters about it is invisible to conscious awareness.

Some of what we know:

We misjudge duration routinely. Healthy adults asked "how long did you sleep?" are off by 30+ minutes on average compared to EEG measurement ( https://www.sciencedirect.com/science/article/abs/pii/S0022399999000173 ). Insomnia patients are often off by 1 to 2 hours in either direction. The subjective "I barely slept" can co-exist with 6+ hours of measured sleep (this has a clinical name: paradoxical insomnia, or sleep state misperception, affecting ~5 to 10% of insomniacs, https://pubmed.ncbi.nlm.nih.gov/29402512/ ).

We can't sense sleep stages at all. You have no conscious access to whether you're in N2, N3, or REM at any moment, except in rare lucid dreaming. You wake up with a feeling. That feeling is dominated by which stage you woke from, not by the night as a whole. You can have an objectively excellent night and feel destroyed (mid-N3 awakening). You can have an objectively broken night and feel fine (woke up during light sleep in the middle of a cycle) https://pubmed.ncbi.nlm.nih.gov/12531174/ .

We miss our own awakenings. The average adult has 10 to 20 brief awakenings per night that last less than 30 seconds. We remember almost none of them. Sleep apnea patients can have 30+ breathing-disruption events per hour, hundreds per night, and be completely unaware of it. Partners often notice before patients do.

We miss our own REM almost entirely. Most dreams are forgotten within a few minutes of waking. The last REM period of the night is the only one we typically remember, and only if we wake up during or just after it. People on SSRIs or cannabis often don't realize their REM has been suppressed for years https://pubmed.ncbi.nlm.nih.gov/28791566/ .

What we can sense, with moderate accuracy:

Roughly how long it took us to fall asleep. Big, memorable awakenings (not small ones). A gestalt "was this a good night or not" feeling the next morning, which correlates with objective sleep around r = 0.3. (Meaning: Roughly 7 out of 10 mornings, your gut feeling and your actual sleep architecture don't match as well as you'd think.) https://pubmed.ncbi.nlm.nih.gov/33530453/

What we can't sense at all:

Total sleep time with precision. How much deep sleep we got. How much REM we got. How fragmented the night was. Whether we stopped breathing. Whether our autonomic nervous system recovered.

The unsettling implication: for most of human history, the only way to know if you were sleeping well was whether you felt okay the next day. That's a lagging, low-resolution, easily confounded signal. Someone can feel "fine" for years while objectively running a significant sleep debt, an undiagnosed apnea, or chronic REM suppression from a medication.

This is one of the reasons sleep is so hard to improve without measurement. You can't optimize a signal you can't perceive. It's roughly analogous to trying to improve your blood pressure by "feeling" it, or your blood sugar by introspection. We don't expect those to work. But we expect ourselves to know how we slept, and we mostly don't.

A note on sleep trackers, since this is where people reasonably look for the missing signal:

I'm not thrilled with the marketing claims of wrist and finger trackers. Tracking is genuinely the only bridge between you and what's actually happening in your brain at night, and because people have no internal feeling for whether their sleep was good, they come to these devices with a built-in expectation of reliability. That's the problem. Reliability on consumer wrist and finger trackers is, honestly, patchier than the marketing suggests, because these devices don't measure sleep. They measure movement (accelerometers) and cardiovascular signals (PPG-based heart rate and HRV), and then infer sleep stages from patterns in those signals. It's indirect measurement ( https://academic.oup.com/sleep/article/44/5/zsaa291/6055610 ). The professionals in a sleep lab measure sleep directly from the brain, using EEG, because sleep is a brain phenomenon. That gap between "inferred from the wrist" and "measured from the brain" is where most of the accuracy loss lives, and the size of that gap is often underappreciated.

Open for discussion, curious what others think.