for me it's 2 consecutive "nights" of 17 hours. That way I'll get about 3 to 4 hours of actual quality awakeness. Doesn't work always though, sometimes I can't fall asleep enough.
Anything below that is always crap, no matter what.
alright fuckers let's go over this again (and spread the knowledge please, it baffles me how it's apparently niche info)
your sleep is on a cycle, each cycle is ~90 minutes, you need from 4 to 6 consecutive cycles to get a good night's sleep, that's 6 to 9h (how much you need depends on your body)
now here's the kicker - if you wake up at the end or the beginning of a sleep cycle, which is when the lightest phase of sleep occurs - you'll feel nice and refreshed.
but if you wake up in the middle - during the deepest phase of your sleep cycle - you'll feel like a fucking zombie and are quite likely to promptly turn off your alarm & go back to sleep and not even register that action as a memory because most of your brain is still asleep
this is also why you can't really have a 40 min nap that leaves you refreshed, you can either have a refreshing <20min nap or a refreshing ~90min nap, anything inbetween will make you feel more tired as your mind now has to wake itself up all the way from deep sleep
so yeah, sleep in ~90min increments, my personal sweet spot is 7:30h
So either i say im going to sleep "now" and it tells me all the times that will align with light sleep and i can choose one to set an alarm for. (It even accounts for the average time it takes people to fall asleep)
Or i can say i want to be up at X time, and it tells me the best times to put my head on the pillow (phone off and eyes closed actually trying to sleep)
Sometimes it works out better for me to get slightly less or more sleep if it means waking during light sleep.
you need 7h or more of sleep regularly or you'll get neural damage in the long run. i'm talking tinnitus, spasms, halluzinations, etc. a few days less per month won't be a problem
to some degree, yes. like hallucinations or eye twitching will usuallly disappear after one or two nights of 7+ hours of sleep. tinnitus can take several days to disappear.
If I'm hitting 10 hours of sleep, it's generally because I've been missing out on sleep, though I generally feel great after that 10 hours. If I get enough days in a row where I can sleep uninterrupted, then I seem to settle at around 8 hours.