I just remembered, during my university years, I used to live comfortably with just 4 hours of sleep. I couldn’t quite understand how, but if I went to bed at 6 PM, no matter what, I would always wake up at 10 PM feeling fully rested. It wasn’t sustainable, though, because I’d want to do something in the evening, and sleeping from 6 PM to 10 PM didn’t seem appealing. However, during exam weeks, it made total sense to sleep only 4 hours and study for the rest of the day. So, I would sleep just 4 hours throughout all exam weeks, and I didn’t have any trouble with being tired or anything, but only if I slept between 6:00 PM and 10:00 PM. It’s really strange.
I had said that 4 hours of sleep was insane, but I take it back. I had very productive days with just 4 hours of sleep.