I wonder at which point in time did we stop going to sleep when we were tired and just started subscribing to some random number like 8 hours of sleep per day.
The Industrial Revolution, probably. You can't simply "go to sleep when you're tired" when you have to allocate x hours to labor per day - everything you do is synchronized to either education hours or work hours for most of your life. So 8 hours of sleep with 8 hours of work and 8 hours of leisure per day.
Humans are so stupid.