American high schools frequently start earlier than elementary schools, at the age when the internal clock has shifted latest. The mismatch is biological before it is behavioral.
The body clock shifts later during adolescence
Through puberty the timing of sleepiness moves later, so a teenager who is not tired at ten at night is displaying a developmental change rather than defiance.
The shift appears across cultures and is observed in other mammals, which argues for a biological driver rather than screens or social life alone.
It also reverses in the twenties, which is why the same person becomes an earlier riser later without any deliberate effort.
Sleep pressure and clock timing pull in opposite directions
Adolescents accumulate the drive for sleep more slowly across the day, which allows them to stay alert later into the evening than younger children.
Meanwhile the timing signal that would produce sleepiness has moved later, so both systems permit a later bedtime while the alarm stays fixed.
The need for sleep itself does not decrease during these years, so a later bedtime with an early alarm produces a shortfall rather than a preference.
Bus scheduling sets the bell, not pedagogy
Districts commonly run the same buses in tiered routes, serving one school level and then another, which requires staggered start times.
High schools are usually placed in the earliest tier because older students are considered able to manage a dark walk and an early stop.
Athletics, after-school jobs and childcare arrangements for younger siblings are then built around that bell, which makes the schedule expensive to move.
Later starts have been tried and studied
Districts that have pushed start times later have been examined for effects on attendance, tardiness and self-reported sleep duration, with generally favorable findings.
Some states have legislated minimum start times for secondary schools, which shifts the question from individual district negotiation to statewide implementation.
Implementation is where most of the difficulty sits, since transportation contracts, athletic conferences and family schedules all have to move together.
What families can influence and what they cannot
Morning light exposure and a consistent wake time help anchor the clock, while bright light late in the evening pushes it further in the unhelpful direction.
Weekend catch-up sleep partially repays the debt and simultaneously delays the clock further, which is why Monday is reliably the hardest morning.
Persistent daytime sleepiness despite adequate opportunity is not normal adolescence, and it warrants evaluation rather than a stricter bedtime.