Yeah like skibum said you will usually wake naturally when you've had enough sleep, but also a healthy sleeping schedule can really help, before I had my son I used to stay up all night and stuff and I felt rubbish in the morning, but after I had to follow his sleep routine a bit and I feel fine not even tired when I wake up now
I think also looking at your screen for a long time and especially in the dark, no matter how much sleep you get can make you feel a bit off
Another thing I'd note is if you feel better on less sleep it could be because that's what your body is used to but then you could still be sleep deprived, so night's when you get 8 hours, you wake feeling off because you technically still are sleep deprived and your body isn't used to sleeping so long anymore
Routine is definitely key
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Crazy cat Lady with a crazy little boy <3