Teen sleep: why everything changes

Why does a teenager go to bed so late?

At puberty, the brain starts releasing melatonin later in the evening, often an hour or more later than in childhood. This marker, the time melatonin onset begins under dim light, is correlated with pubertal development stage: the further puberty advances, the later it shifts. That holds even in teenagers whose lab sleep schedule stays strictly identical from one stage to the next. The shift therefore comes from the maturation of the internal clock, not from preference or slack discipline.

Two other mechanisms add to it. A teenager's own circadian rhythm runs a little over 24 hours on average, which naturally favors falling asleep a bit later each day. And sleep pressure, the tiredness that builds over the hours awake, rises more slowly in late-puberty teenagers than in younger children: they can stay awake longer in the evening without feeling the same drowsiness.

The conflict with school schedules

The conflict comes from teenagers still needing a lot of sleep: the American Academy of Sleep Medicine recommends 8 to 10 hours per 24 hours between ages 13 and 18 (see our guide on sleep by age). That need doesn't drop as much as people think. According to a review by Carskadon (2011), the loss of sleep in adolescence doesn't come from a lower need, but from a convergence of biological, psychological, and sociocultural factors. With a shifted internal clock and an early school wake-up, that benchmark becomes hard to reach.

In practice, for high schoolers whose classes start at 7:20, the biological night often runs until the second class of the morning. Several doze off or slip into light sleep during tests: it's the expected consequence of a clock moving back while the alarm moves forward.

The September back-to-school, by contrast, mostly calls for resetting a rhythm that drifted over the summer (see our guide on sleep and back-to-school). The conflict described here is different: it replays every school morning for as long as adolescence lasts, not just during a one-off transition.

School start time, a lever families don't control

The American Academy of Pediatrics identifies school start times before 8:30 am as a major modifiable factor in middle and high schoolers' lack of sleep. It treats insufficient sleep at this age as an important public health issue, and urges schools to aim for start times that give students the opportunity to sleep 8.5 to 9.5 hours. It also notes that delaying school start times has been shown to work against chronic sleep loss.

Families don't control the high school timetable. They can, however, keep in mind that morning drowsiness isn't bad will, and act on what is up to them: the weekend wake-up time, morning light, the end of the evening.

Weekends: a slide that makes Mondays harder

On weekends, bedtime and wake-up time slide noticeably, with no school alarm to hold them. The gap between weekday and weekend wake-up time averages 3 to 4 hours among high schoolers, compared with 1.5 to 3 hours among younger middle schoolers. That gap, repeated from one weekend to the next, sustains the late rhythm rather than softening it.

Keeping a wake-up time within an hour of school days limits this slide without ruling out the occasional lie-in. It's the simplest lever to avoid starting every Monday with a rhythm even more shifted than the previous Friday.

Take a high schooler who gets up at 6:45 on weekdays and 10:30 on weekends. On Sunday night, their clock has gotten used to a late wake-up: they don't fall asleep until well past midnight, and on Monday morning the alarm rings in the middle of their biological night. If they get up around 7:45 on Saturday and Sunday instead, the gap with Monday shrinks to an hour, and Sunday night gets a bit easier.

What helps, without fighting biology

Forcing an earlier bedtime through constraint alone rarely works at this age: until melatonin has kicked in, falling asleep doesn't happen, forced or not. What helps more is acting on both ends of the day at once, with a stable wake-up and morning light exposure, rather than a bedtime imposed in the evening.

Morning light (opening the curtains, getting outside early, eating breakfast near a window) helps the internal clock move earlier faster than an early bedtime alone. A stable end-of-evening routine, even without sleep right behind it (reading, a shower, dim light), sets the stage so falling asleep comes as soon as melatonin starts, rather than delaying it further.

Evening screens: a factor that makes things worse, not the only cause

A screen on in the evening pushes back falling asleep further, through the light it emits and through content that keeps attention awake just when the body should be slowing down (see our guide on screens and sleep). But removing screens isn't enough to cancel the shift: a teenager's melatonin would start later than a child's even with no screen at all in the evening.

The two causes add up rather than exclude each other. Cutting screens an hour before bed removes some of the delay, but the biological base of the shift remains underneath. Treating screens as the only problem often leads to evenings of conflict that don't move falling asleep as fast as hoped.

When should you worry about a teenager's sleep?

A late bedtime and a hard time waking are part of ordinary teenage sleep, but some signs justify talking to a doctor. That's the case if daytime sleepiness persists despite enough nights, if the teenager falls asleep in class, if their mood changes for a lasting stretch, or if grades drop with no other explanation.

Sleep that seems clearly too short or too long is also worth raising. Before the appointment, two weeks of a sleep diary (bedtime, time falling asleep, and wake-up, on weekdays and weekends) gives the doctor a more precise benchmark than memory. This guide doesn't replace medical advice.