[CleanCoder]

During my University days I've found out that I feel rested when sleeping for a number of hours divisible by 1.5, or to put it simply, only multiples of 1.5h. So, if I had to take a nap or work super late a 3h "nap" would've left me fresh and ready to work while a 3.5h or 4h nap would turn me into a zombie. I haven't thought of this in a while since it became a habit for me but seeing the 7.5h you mention along with small time changes making big difference it all came back.

[ianai]

People sleep in cycles. Everyone’s timing of a cycle is different. But the average is around 90 minutes. You’ll feel dramatically better keeping your sleep to obey the boundaries of those 90 minute cycles.

Also REM occurs in the last cycles of sleep. So sleeping 4 hours with little REM and sleeping 6.5 hours with REM is like night and day.