I’ve learnt a little about sleep cycles and remember that every 90 minutes is when we move through the 5 stages of sleep, but I’ve also heard it’s recommended to have 8 hours of sleep. Wouldn’t that make you feel drowsy if you’re interrupting the sleep cycle when you wake up after 8 hours?
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The idea of an 8 hour sleep cycle came around the time Henry Ford was changing labor practices. (For his own benefit but it did help some people.) Now the unions at the time were fighting for better work hours. This was the “2 cents for pouring metal in a factory 80 hours a week era.” One of the BIG slogans for workers rights was “8 hours of work, 8 hours of Leisure,and 8 Hours of sleep.” As an example of a fair work day for employees. It worked and it stuck.
There is no evidence saying people NEED a full 8 hours. Indeed everyone is different. I sleep best with 6 most nights. I have friends who do two four hour sleeps. I even have friends who just “sleep when they are tired.” In fact, up through Puritan times the norm was to sleep early. Wake up in the middle of night for a couple hours then sleep again till daybreak.
Sleep is mysterious piece of human physiology we don’t fully comprehend.
TL;DR: You don’t, industrial era workers lobbied for a more fair work day and “8 hours for sleep.” Was one of their demands that’s persisted through the years.
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