The National Sleep Foundation warns that Americans aren't getting enough sleep, in part because they use electronic devices too close to bedtime. Adolescents are the most sleep-deprived, getting just 7 hours and 26 minutes a night, when they supposedly need 9 hours and 15 minutes. But how do scientists know precisely how much sleep teenagers need?
By leaving them alone. Scientists still aren't entirely sure why we sleep, which makes it very difficult to develop recommendations for how much to sleep. But in a classic 1980 study, Mary Carskadon of Stanford sequestered a group of adolescents in the university's sleep laboratory for several days, letting them sleep for as long as they wanted, up to 10 hours. She found that the teenagers slept just over nine hours, with very little variation. This so-called "naturalistic" study is the primary basis for the adolescent sleep recommendation.
Recommendations for adults are often based on surveys. Researchers look for correlations between people's self-reported sleep habits and their real-world behavioral patterns. Adolescents who sleep fewer than eight hours, for example, tend to get lower grades. Adults who sleep fewer than seven hours self-report memory loss and difficulty concentrating.
Alternatively, adult recommendations may be derived from cognitive performance studies. Researchers sleep-restrict a group of participants, then compare their performance on a task to a well-rested control group. The test-takers may have to keep a cross within a moving square on a computer screen using a mouse, or punch a key when certain letters appear on the screen, or navigate a maze. Other investigators prefer reaction-time tests or driving simulators. The results of such studies vary, but they generally suggest that adults sleeping fewer than six hours suffer cognitive impairment.
Studies on infants and young children are more problematic, because babies can't take cognitive tests or fill out surveys. The recommendation that infants sleep between 12 and 18 hours a day is based on a combination of uninterrupted sleep studies, like Carskadon's experiments on adolescents, and parental questionnaires concerning the baby's sleep and how fussy it is when awake. But many researchers find the 12-to-18-hour range very speculative.
A small but vocal minority of researchers rejects these approaches entirely, arguing that none of them have to do with health. Just because teens sleep nine hours when left alone doesn't mean it's the best thing for them, just as letting people eat as much as they want isn't good for their health. And conquering a maze doesn't pertain to, say, long-term cardiovascular fitness. Besides, being a little sleepy at two in the afternoon isn't necessarily unnatural or bad for you, unless you're operating heavy machinery. (Gorillas and chimps in the wild often drift off for a bit during daylight hours.) According to these skeptics, the important issue is whether prolonged sleep extends life, or if too little sleep shortens it. Studies by Daniel Kripke of UCSD, for example, show that adults who sleep between six and seven hours per night live the longest, while those who sleep fewer than 5 hours and those who sleep more than 8 hours die younger.
These longevity studies are quite controversial, because it's not clear whether excessive sleep is the cause of ill health or merely a symptom. For example, people who suffer from depression, obesity, or cardiac disease may sleep longer than their healthy peers. But this critique could be leveled against many sleep studies. Adolescents who sleep less may live in chaotic households that are the true cause of their poor school performance. And chronic moodiness may cause sleep deprivation, rather than the other way around.
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Explainer thanks Daniel Kripke of the University of California, San Diego, and Amy Wolfson of the College of the Holy Cross and the National Sleep Foundation.