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