For real though, it’s habit. They didn’t naturally start that way. They just kept doing it that the body eventually “learned” to set their sleeping rhythm to that schedule.
Nope, they’ve always been that way, no training required. My husband can go to bed at 4am and will still wake up when the sun comes out.
If left to my own devices, I run on about a 28 hour day and will eventually go fully nocturnal. That said, I have trained myself to go to bed at 2 and wake up at 10. But if I try to go to bed at 9pm so I can wake up at 5am, all that will happen is that I toss and turn until 2 am, then wake up at 5. No amount of training will fix that. I averaged 4 hours of sleep per night back when I was in school, simply because I could not go to sleep that early.
So while training is absolutely possible, I think you’re underestimating the power of the natural circadian rhythm and how people react to it.
Everyone has a different natural circadian. Sure you can force yourself to go to bed at a time when your body doesn’t want to and try to “train” yourself, but really you’ll just be tired all the time. Your body does not learn a new time, you just get used to being tired.
