The Sleep Shift

If you’ve ever lain in bed at 10 p.m., tired but wide awake, you’ve met a real piece of biology. Sleep is timed by two systems working together (Crowley et al., 2007).

The first is your body clock: a roughly 24-hour rhythm that sets when your body’s night begins — one sign is a rise in the hormone melatonin in the evening. The second is sleep pressure: a drive to sleep that builds the longer you’re awake and drains while you sleep. When both say “night”, sleep comes easily.

Around puberty, both shift. The clock’s night starts later, and sleep pressure builds more slowly across the day (Carskadon, 2011). So at 10 p.m. your body may simply not be ready yet — and your alarm hasn’t moved at all.

Here’s the puzzle. Teens get more homework, more freedom and more phone time at the same age their sleep moves later. So how do you tell biology apart from lifestyle?

One way: compare young people of the same age at different stages of puberty (Carskadon et al., 1993). Another: bring teens into a sleep lab, control everything, and measure sleep pressure directly (Jenni et al., 2005). A third: measure how the clock reacts to light, using melatonin in saliva (Crowley et al., 2015).

No single study settles it; each has a weakness the others cover. That’s how a strong scientific case gets built: different methods, pointing the same way.

Screens are the usual suspect, and the link is consistent: a review of 67 studies found screen use tied to poorer sleep — mostly later or shorter — in 90% of them. But almost all were observational, so the review couldn’t confirm that screens cause the problem (Hale & Guan, 2015). Maybe screens delay sleep; maybe teens who can’t sleep reach for their phones. Possibly both.

Researchers describe three ways screens could get in: they take up time that could have been sleep; what’s on them — a game, a group chat — can leave you wound up; and their light can push the clock later (Hale & Guan, 2015). Interactive screens were tied to poorer sleep a little more consistently than TV, which fits the idea that what you do matters, not only the light.

Light’s effect is real, but dose matters. In a lab, 12 young adults read on a tablet at full brightness for about 4 hours, five evenings in a row. Compared with a paper book, their melatonin onset came more than 1.5 hours later and they took about 10 minutes longer to fall asleep (Chang et al., 2015). That was a big dose — hours of full brightness — and in adults, not teens.

You can’t will your clock earlier. But you can stop pushing it later, and nudge it the right way.

Light is the clock’s main time signal (Crowley et al., 2015). In teens aged 14 to 17, bright light around their usual bedtime pushed the clock later, and light around their usual wake-up time pulled it earlier (Crowley & Eastman, 2017). So dimmer evenings and daylight soon after you get up can help.

Steadier wake-up times can help too. Social jet lag puts your body in two time zones (Wittmann et al., 2006): in the Seattle study, students fell asleep later and slept less on Sunday nights than on Monday nights (Dunster et al., 2018). Keeping weekend wake-ups within reach of school days shrinks the jump.

And the phone? Its effect seems to depend on dose, timing and what you do on it — and nobody has measured it in you. That’s what your experiment is for.