u/RedditPotato54

I've been looking at sleeping bags for winter camping in the mountains, specifically for conditions between -10° and 0°C and theres plenty of bags that are rated for just that, but do they take into account sleeping in heavy clothes or just thermal layers? If it's -10° and I'm sleeping in a 0° comfort bag, do I need anything more than a thermal top, bottom and a sweater or how does it work? (Assuming I have a let's say 5R value mat setup) How much more do I need to compensate if my R value is say, 3.5?