When It Comes to Sleep, One Size Fits All

When It Comes to Sleep, One Size Fits All

A massive new study shows that every adult needs 7-8 hours a night, or else their cognitive abilities will suffer

When It Comes to Sleep, One Size Fits All

I’d always thought that our need for sleep, like our appetite for food, drink or social contact, was a personal matter: Some people need more, some need less. Age, lifestyle, work and metabolism combine to determine how much sleep a person needs to function, and if some people thrive on five hours a night and others require seven, chalk it up to different strokes for different folks, right?

Wrong. A new study of the sleep habits of more than 10,000 people around the world suggests that the amount of sleep adults need is universal. The massive survey, published in the journal Sleep, demonstrates that adults everywhere need 7-8 hours a night—no more and no less—in order to be mentally limber. When we habitually stint on those hours, higher-order cognitive processing—such as the ability to see complex patterns and solve problems—is compromised.

The lead author of the study, Conor Wild, a research associate at the Brain and Mind Institute at the University of Western Ontario, explained how a research team directed by the neuroscientist Adrian Owen recruited thousands of people for the online study. “We did a lot of social media advertising and radio interviews. And the BBC put together a two-minute spot on Dr. Owen’s sleep research,” Dr. Wild said.

This blitz prompted more than 40,000 people from 150 countries to sign up. Ultimately, two thirds of them were eliminated due to technical glitches, incomplete questionnaires, or extreme responses—such as reporting that they sleep zero hours a night, or more than 16.

The 10,886 adults who remained filled out a detailed online questionnaire about their backgrounds, medical histories and sleep patterns. How long was their average night’s sleep? How often was their rest interrupted, and how consistent were their nights?

Once their sleep habits were recorded, the participants completed a battery of 12 cognitive tests. Puzzle-like tasks assessed their spatial, verbal and short-term memories, as well as their capacity for deductive reasoning, sustained attention, planning and clear expression.

The findings that emerged were startling. Half the sample averaged less than 6.4 hours of sleep a night—a pattern that was associated with impaired problem-solving, reasoning and verbal acuity. Those who routinely slept six hours a night or less flubbed more questions based on spatial rotation or grammatical reasoning and left more tasks incomplete than those who got a full night’s rest.

Surprisingly, mere sleep deprivation—that is, one or two nights with little or no sleep—did not alter reasoning or verbal skills, though it did hobble short-term memory. This finding is reassuring, given that many professionals—think of hospital residents and airline pilots—have to make life-or-death decisions based on exactly this kind of erratic sleep schedule. If their short-term memory is compromised, they can always look up facts on their phones. But for split-second, life-changing decisions, they are on their own.

Regularly sleeping too little seems to be much more damaging than having one or two bad nights. Getting four hours of sleep or less for an extended period is equivalent to adding eight years to one’s age when it comes to test performance, the study shows—a significant decline. But a single good night can repair some of the damage: People who slept more than usual the night before they were tested ended up acing more of the cognitive tests.

So if you want to be articulate, solve a pesky problem, parallel park or organize an effective team—or even your closet—you’ll definitely need that nightly 7-8 hours of sleep.