I get that there are a lot of valid cases where the answer is "homework", "circadian rhythms", "evolution", but...
80% of the answer here is electronics, and really just social media + smartphones. It's incredibly easy to stay up too late when you're hyperconnected and chatting with your classmates late into the night. It just wasn't even a possibility 20 years ago.
Kids and teenagers need some time to do their own thing, so given that most of the "waking hours" are filled with school and other things that we effectively force on them, of course they try to steal some hours for themselves where they can.
In my case, it was programming 8 bit computers and yakking on the CB net a friend and I had setup. We had about 20 people on it at one point. Adults tuned in regularly to listen to us kids talk about all kinds of shit.
Amazingly, they left us alone.
One of the best things we adults can do for our kids, a lot of the time.
The other thing we can do is bring them a lot of hands on experiences. Doing stuff.
Both are high value.
I stayed up until 2 or 3 am reading books because I wasn’t tired. They’re staying up chatting with their friends because they’re not tired. The connection is the “not being tired.”
After lunch, we went out to help my father build houses, then off to sports practices (typically two separate practices for two different sports) and classes at the local college. By the end of the day you're exhausted!
A large part of why my parents were so strict is because we were really poor and my parents were determined to not only get out of poverty but also to teach us a good work ethic.
So part of me is really biased in thinking "these kids have an easy schedule, why is it so hard for them to get going in the morning?"
I wouldn't recommend my upbringing necessarily, but also can't help but wonder if kids had a stricter routine, regularly exercise, good diet, and family support if it wouldn't be slightly easier for them to start the day early.
But also on the flip-side I think it's great to let them sleep in because I remember how I felt, and how my friends felt.
Lastly, perhaps public education is partially at fault? Sitting in a classroom for hours every day is exhausting, so how about a more hands-on method of learning where kids actually stand at a white board and write out problems and solutions - something that might work in the lecture part of chemistry, physics, math, and perhaps other courses? - We did this in homeschool as well as at the local college I attended and it absolutely helped me wrap my head around things better!
I ultimately have no dog in the fight as my family and I do our own thing, this is just my opinion.