School start times and screen time late in the evening exacerbate sleep deprivation in US teenagers

With the school year underway around the U.S., parents and caregivers are once again faced with the age-old struggle of wrangling groggy kids out of bed in the morning. For parents of preteens and teenagers, it can be particularly challenging.

Sometimes this gets chalked up to laziness in teens. But the main reason why a healthy person is unable to naturally wake up without an alarm is that they are not getting the sleep their brain and body need.

That’s because studies show that adolescents need more than nine hours of daily sleep to be physically and mentally healthy.

But the likelihood that you know a teenager who gets enough sleep is rather slim. In the U.S., less than 30% of high school students – or those in grades 9 through 12 – sleep the recommended amount, according to the Centers for Disease Control and Prevention. Among middle schoolers in grades 6-8, nearly 60% do not get enough sleep at night.

Yet my laboratory’s research suggests that a much higher percentage of teens are getting too little sleep.

I am a professor of biology and have been studying sleep and circadian rhythms for more than 30 years. For the past seven years, my laboratory at the University of Washington has been doing research on sleep in Seattle-area teenagers. Our research has found that, just as in other areas of the U.S., high schoolers in Seattle are not getting the amount of sleep they need. Our study objectively measured sleep in 182 high school sophomores and seniors and found only two that slept at least nine hours at night during school days.

Our studies and those of others indicate that three important factors lie behind this lack-of-sleep epidemic: a physiological regulation of sleep that leads to a delayed sleep timing in teens and that is not aligned with early school start times, a lack of morning exposure to daylight and excessive exposure to bright electric light and screens late in the evening.

Teen sleep biology

The time people go to bed, fall asleep and wake up is governed by two main factors in the brain. The first is a so-called “wakefulness tracker,” a physiological timer that increases our need to sleep the longer we stay awake. This is in part the consequence of the accumulation of chemical signals released by neurons, such as adenosine.

Adenosine accumulates in the brain when we are awake, leading to increased sleepiness as the day wears on. If, for instance, a person wakes up at 7 a.m., these chemical signals will accumulate throughout the day until the levels are high enough that the person will fall asleep, typically in the late evening.

The second factor that drives the sleep/wake cycle is a 24-hour biological clock that tells our brain what times of the day we should be awake and what times we should be sleeping. This biological clock is located in an area of the brain called the hypothalamus. The clock is composed of neurons that coordinate the brain areas regulating sleep and wakefulness to a 24-hour sleep/wake cycle.

These two regulators operate with relative independence from each other. But under typical conditions, they are coordinated so that a person with access to electric-powered light would fall asleep in the late evening – between about 10 p.m. to 11 p.m., and wake up in the early morning, around 6 a.m. to 7 a.m.

So why do teenagers often want to go to bed later and wake up later than their parents?

It turns out that during adolescence, both the wakefulness tracker and the biological clock conspire to delay the timing of sleep. First, adolescents can be awake until later hours before their wakefulness tracker makes them feel sleepy enough to fall sleep.

Second, the biological clock of teenagers is delayed because in some cases it appears to run at a slower pace, and because it responds differently to light cues that reset the clock daily. This combination leads to a sleep cycle that operates a couple of hours later than in an older adult – if an older adult feels the signals to fall asleep around 10 p.m. or 11 p.m., this won’t happen until midnight or later in a teenager.