Page 92:
"Adolescents face two other harmful challenges in their struggle to obtain sufficient sleep as their brains continue to develop. The first is a change in their circadian rhythm. The second is early school start times."
Also:
"... the circadian rhythm of a young child runs on an earlier schedule. Children therefore become sleepy earlier and wake up earlier than their adult parents. Adolescent teenagers, however, have a different circadian rhythm from their young siblings. During puberty, the timing of the suprachiasmatic nucleus is shifted progressively forward: a change that is common across all adolescents, irrespective of culture of geography."
It goes on to explain why this is the case, and the impact on your sleep schedule.
Also page 308-16 there's an entire chapter on Sleep and Education, and the problem of schools starting progressively early, mostly for the convenience of society at large, rather than a net benefit of the kids.
The book has been largely discussed on HN before; definitely worth reading.
[1] https://www.amazon.com/Why-We-Sleep-Unlocking-Dreams/dp/1501...