>Early school starts are not the only cause of teenage drowsiness, but they are a crucial factor—especially because natural sleep cycles make it difficult for post-puberty teenagers to fall asleep before 11 p.m.
The answer is (surprisingly to most) complicated and illusive.
Laying down in bed at 9pm doesn't guarantee a person falls asleep right away. For some people, yes, their natural cycle syncs up with an ideal work/life sleep schedule, or their bodies are just better at adapting.
But that isn't the case for everyone.
For example, the change in an animal's coat type (summer vs winter) is triggered by the number of hours of sun they are exposed to. Once the days get shorter, the summer coat falls out and the winter coat starts growing in.
Presumably that means someplace your body has the equivalent of an accumulator running measuring hours of sun exposure. :-D (This is also why people deprived of light, or exposed to non-stop light, quickly find a lot of their body's rhymes get way out of wack.)
Anyway AFAIK that system is part of how we adjust, eventually, to changes in time zone.
A lot of other things in our bodies are cyclical throughout the day, month, and seasons. Including men's hormone levels, with testosterone "peaking in October and ebbing in April" [https://www.apa.org/monitor/2011/03/hormones#:~:text=Men%27s....]
I guess my TL;DR is that we're still animals and we can't logic / alarm clock our way out of biology.
People's daily clock is naturally (i.e if kept in a dim light constantly) slightly longer than a day (exactly how long varies by a few hours), but it becomes synchronised to changes in light level (there are actually specific light receptors in your eyes that do this, independent of vision). Exactly what 'phase' this clock has compared to the change in light levels varies from person to person, and throughout their life. There's a decent body of evidence that teenagers naturally shift towards being more delayed as they go through puberty. There are even disorders where it can become very far out of whack: people who will want to sleep very early or very late, or in the most extreme case (usually people who are totally blind in a way that also disrupts these specialised receptors, but also some people who have functioning sight), where the synchronisation doesn't work at all and their natural sleep cycle will drift freely with respect to the day-night cycle.
depending on which physician you ask, it can actually be worse to just lie in bed for hours and hours without sleeping.
it's definitely bad to put yourself in a situation where you must go to sleep at X PM and wake up at Y AM, and then you're repeatedly failing to fall asleep so now you rely on things like sleeping pills, or you're perpetually exhausted and underperforming at tasks like classes or work and the stress creates a feedback loop of even worse sleep.
it certainly wasn't good for me when i was younger!
This gets even more important for kids and teenagers because your chronotype can shift drastically as you transition towards young adulthood until it settles into a certain rhythm.
I have 2 children, school starts at 845 and it's within a 15 minutes walk. We get to sleep until 745,which is great.
Our bedtime is always the same 8 in bed, about 830 asleep and for the adults between 1030 and 11 (I need 9 hours). As you can see pretty consistent sleep schedule.
My sleep is somewhat disturbed, as in I wake up a minute 2-3 times per night. Not alwaya, but frequent.
Last time with new year (no alcohol), I went to sleep at 3 am, and I forgot how good is it. I'm a former night owl, at 3 am ny body feels tired, it takes zero effort to sleep, I closed my eyes and I passed out, deep sleep for 6 hours, the use washroom and another 2 hours of sleep.
It was incredible. Society doesn't allow that, but the difference between "have to go to bed" and "go to bed when ready" is huge
¹(sunrise ~7:11a in the winter, we'd wake up at 6a. ~40m for breakfast/morning routine, ~1h for transit, and a little bit of wiggle, since you'd want to be there by ~7:40a at the latest.)
(… and there are routinely threads on HN where people advocate for moving that wakeup to ~5a standard by futzing with DST.)
When I was a teenager the TV shows I wanted to watch usually dictated when I went to bed.