The following originally appeared on The Upshot (copyright 2016, The New York Times Company).

As a pediatrician, I find that there are few topics that parents want to discuss more than sleep. Parents worry about their own sleep deprivation when babies arrive. Later, they worry about their children’s. I almost never encounter patients who are convinced that they’re getting the recommended amount of sleep.

It’s harder than you might think to determine how much sleep an adult actually requires. Modern technology has significantly altered how and when we might naturally sleep. Electricity allows us to be productive long after the sun has gone down. Coffee and other stimulants allow us to wake up more quickly. Measuring “natural” levels of sleep would require us toreturn to a simpler time.

As part of a German science television show, five men and women volunteered to return to Stone Age conditions for eight weeks. They had no smartphones or Internet, no electricity or running water, no alarm clocks — or any clocks for that matter. Enterprising scientists took advantage of this to make some measurements.

Before the study, they went to sleep (median) about 20 minutes before midnight. Without interference from modern amenities, their bedtimes moved up about two hours. Before the experiment, they woke up (median) about 7 a.m. Under Stone Age conditions, they woke up about a half-hour earlier.

Counting the periods of awake time between going to sleep and waking up in the morning, they had been spending less than six hours asleep each night before the experiment, and without outside interference they slept about seven and a quarter hours a night. This might be the closest we’ll get to figuring out what a modern human body naturally requires.

Granted, many people probably aren’t getting that much. In 2013, the National Sleep Foundation released the results of a survey on sleep among people 25 to 55 in six countries. Canadians and Mexicans topped the list at 7.1 hours a night, followed by Germans at 7 and residents of Britain at 6.8. Bringing up the rear were Americans at 6.5 hours and Japanese at 6.4.

Sleeping hours are disproportionate across the socioeconomic spectrum as well. The more you make, the less you sleep. Almost half of people earning less than $30,000 a year sleep at least six hours a night, compared with about a third of those earning at least $75,000.

Americans also sleep less than we used to. In 1942, almost 85 percent of us slept at least seven hours a night. Today, less than 60 percent of us do.

Not getting enough sleep is a big problem. Randomized controlled trialsshow that people who are sleep-deprived can see decreases in their empathy. More than one such study has shown that sleep deprivation can leave people more sensitive to pain. Sleep deprivation can hurt cognition, and it is associated with many, many car accidents.

But serious sleep deprivation in adults is most likely rarer than many think it is. After all, people in controlled studies of sleep deprivation are usually getting very, very little sleep. Complicating things, not all people react to sleep deprivation in the same way. Some people just need less sleep, and that may be somewhat genetic. Many news reports that highlight the dangers from too little sleep are assuming that all adults need at least eight hours. There’s just little evidence that’s so.

There’s one group where that may not be true, however. Younger people need more sleep than adults. The National Heart, Lung and Blood Instituterecommends that newborn babies get 16 to 18 hours of sleep a day. It’s likely that many of them get that, because we let them. As I tell parents all the time, only a fool wakes a sleeping baby. The usual recommendation for preschool children is 11 to 12 hours, school-age children 10 hours and teenagers about 9 to 10 hours a night.

It’s likely few teenagers are sleeping that much.

The most obvious reason for that is that the high school day generally starts so early. Next year, when my oldest heads to ninth grade, his bus will come for him around 6:45 a.m. To get nine hours of sleep, he will have to be asleep by 9:15. Going to bed early doesn’t seem to bother Jacob much, so I imagine he might just do that; most teenagers can’t, though.

Many of them are engaged in activities after school. They eat dinner late, so that they can be with their parents, who probably work late. They also need time to get their homework done, let alone to have any type of social life.

There’s no good reason school has to start this early, and starting it later might improve the amount of sleep teenagers get. A study published in 2014examined 9,000 students in eight public schools in three states. It found that in high schools where classes began at 7:30 a.m., about a third of children got at least eight hours of sleep a night. If they started at 8:35 a.m., about 60 percent of children achieved that goal.

Moreover, the later start time was associated with improvements in a number of subjects, as well as state and national achievement test scores. Attendance increased. Perhaps more important, the number of car crashes by drivers 16 to 18 was reduced by 70 percent when school start times were changed from 7:35 to 8:55.

It’s for reasons like these that the American Academy of Pediatrics released a policy statement in 2014 calling for a shift in school start times to 8:30 or later. Few school systems, however, have heeded the call.

Many media stories about sleep breathlessly worry that the average American is at grave risk because of sleep deprivation. Even if it were true, that could be improved for many of us by choosing to turn off our devices and shut our eyes just a little bit sooner. Too few stories focus on those who are really at risk for sleep deprivation, namely teenagers. It’s not their fault. We could fix this problem for them.

@aaronecarroll

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