I’ve heard my whole life that people need 8 hours of sleep a night to feel good and fully rested. But is that the truth?

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If you’re an American who lives to be 78.8-years-old (the average lifespan in the U.S.), and you get 8 hours every night, you’ll sleep away over 9,500 days during your life. That’s a lot of shuteye. Obviously sleep is important for our overall health, but a lot of stuff gets in the way of those 8 hours: work, kids, the internet, etc. So how much sleep do we actually need in order to function, and can we train ourselves to get by on fewer than 8 hours?

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Well…no. Scientific sleep studies have shown that 8 is truly the magic number. Even when research subjects were given more time in sleep labs, they typically only snoozed for 8 hours. In a pretty extreme sleep study conducted in 1932, a researcher and one of his students (I don’t think this would fly today) spent 32 DAYS living in a cave in Kentucky that is one of the longest and deepest in the world. The cave had no sunlight whatsoever, yet the two subjects slept 8 to 8-and-a-half hours each night.

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But what happens when we just don’t have the time to reach that goal? David Dinges and Gregory Belenky, two sleep researchers, conducted studies to see how little sleep people can get without their cognitive performance being affected. The subjects were broken into groups in which they allowed to sleep a different number of hours each night for two weeks. After only 10 days, the subjects getting 7 hours of sleep each night were as cognitively impaired as the people in the group getting absolutely zero sleep every night.

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The studies surprisingly showed that losing just one hour of sleep per night caused people to become cognitively impaired. But can we make up some of those hours on the weekend if we get behind on our slumber schedule? Dinges and Belenky had their subjects get 3 days of recovery sleep after the study to test that out. Though the guinea pigs slept longer than 8 hours a night during their make-up nights, their cognitive tests showed they still were not back to their baseline levels.

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So if you are sleep deprived, it’s going to take you longer than one weekend to get back to normal. Matthew Walker, director of the sleep and neuroimaging lab at the University of California, Berkeley, says, “People think that sleep is like the bank. That you can accumulate a debt and then hope to pay it off at a later point in time. And we now know that sleep is not like that.” The brain has no storage system for sleep and that’s why sleep deprivation is a hard nut to crack.

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Walker says that many people convince themselves they can get by on less than 8 hours a night, but that they are fooling themselves: “You don’t know you are sleep deprived when you are sleep deprived. That’s why so many people fool themselves into thinking they are one of those people who can get away with six hours of sleep or less.” If you don’t get much sleep, you’re probably just used to feeling a certain way all the time and believing it is normal. Bottom line: 8 hours of sleep is ideal for us humans, but realistically many of us aren’t going to reach that goal. So do the best you can, and try to catch a nap once in a while!

h/t: Popular Science