It's the difference between morning people and night owls.

…even if it were the case that schoolchildren's sleep cycles were dictated by the sun (which several comments in this section indicate isn't the case, and contain references which I suppose one could follow) … my school, for example, started at 7:50am. That meant waking up well before sunrise during some parts of the year (~70 minutes before sunrise, in the darkest portion¹). I remember always being very tired for first period … all the more amazing that I passed AP Physics, which I had first period.

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.)

Sleep Science is ongoing and very much an active area of research. Sleep can be informed by biology, diet, lighting, comfort, heat... among a vast host of other factors. Ask yourself this -- why do you get tired? ("Because I'm sleepy" -- why?)

The answer is (surprisingly to most) complicated and illusive.

We don't all fall asleep simply because we're in bed; some of us, at some stages of our lives, will toss and turn and fidget, waiting for tiredness to arrive - potentially for hours! - if we go to bed earlier than our body clocks demand.

Your entire body runs on an internal clock, down to portions of your immune system that switch over to work differently at night.

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.

This implies that if you simply move time zones sufficiently then your natural cycle would match up with your work/life sleep schedule. Does that actually happen? If not, what other frame of reference is your natural cycle syncing up with?

A lot of biological processes are kicked off by light exposure, and by "number of hours of light exposure".

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.

Light. Especially sunlight, the exact effects of artificial light are not completely clear (or rather, it seems to have an effect, but it's not completely obvious which parts and how: this is part of the theory of why avoiding blue light in the evening can help with sleep but it's not actually been demonstrated that it works as an intervention).

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.

Because the circadian rhythm of a human body can vary drastically from person to person, that's why some people are more naturally night owls, or have cycles closer to 23/25 hours than 24. You can try and make a teenager go to bed at 10pm, but if they're unable to sleep for an hour then it doesn't matter 'when' you go to bed.

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.

Sounds like it's impossible to match everyone's circadian rhythm then, so what can we realistically do about it?

You can at least attempt to aim towards the average and allow for flexibility in it where reasonable, as opposed to viewing the incompatibilty as some moral failing on the part of people who's rhythm does not match up.

I'll throw this in the pool, personal anecdote.

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

Assuming your question isn't sarcastic, laying in bed and struggling to fall asleep for hours doesn't tremendously help with waking up early.

Sunlight

going to bed != falling asleep

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!