"Lost Sleep Is Hard to Find"

It’s a time-honored practice among medical residents, cramming undergrads, and anyone else burning the candle at both ends: get very little sleep for days, maybe even pull an all-nighter, and then crash for an extra-long night of shut-eye to catch up.

Ten hours of sleep at once may indeed recharge us, and allow us to perform well for several hours after waking, according to research recently published in Science Translational Medicine. But “the brain literally keeps track of how long we’ve been asleep and awake—for weeks,” says Harvard Medical School (HMS) neurology instructor Daniel A. Cohen, M.D., lead author of the study. And that means that the bigger our aggregate sleep deficit, the faster our performance deteriorates, even after a good night’s rest.

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Cohen’s study also revealed valuable information about how circadian rhythms influence our responses to sleep deprivation. The researchers determined that hitting the body’s circadian high (from about 3 p.m. to 7 p.m. for most people, when levels of the hormone melatonin are lowest in the bloodstream) can effectively mask the effects of sleep loss on performance—suggesting why chronically sleep-deprived individuals may not feel very sleepy for much of the day and think they’re sufficiently rested. To make matters worse, Cohen says, “Prior research shows that people start to overrate how they perform when they’re chronically sleep deprived.” But the inevitable circadian low (roughly 3 a.m. to 7 a.m., when melatonin levels are highest) magnifies the effects of sleep loss, slowing reaction times by a factor of 10—one reason overnight drivers, for example, are especially prone to errors.

Scientists don’t yet know how long it takes to overcome a long-term sleep debt. “It certainly takes longer than three days,” Cohen says. “It could even take up to a couple of weeks of a normal sleep schedule before people are fully caught up”—an important fact for people with safety-sensitive jobs to know, so they can make adequate sleep a priority. He admits some trouble with this himself, especially during his medical training. “But since I’ve been in the sleep field,” he says, “I’ve tried to shoot for closer to eight hours per night.”