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How much sleep do you really need? A sleep scientist explains - Vox

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Julia Belluz: So is seven hours really the new magic number for sleep?

Hans Van Dongen:  I think it's very tricky, if not outright misleading, to tell people a number for sleep duration that may or may not be best for them and that may in fact cause them short-term cognitive impairment or long-term health consequences or, on the flip side, to struggle to get more sleep that they may not actually need. I was a bit shocked to read that in an age of personalized medicine, advocacy groups are embarking on a mission to recommend one specific sleep duration—especially when that duration hasn't even been firmly established for the average person, let alone different individuals.

We also know that the amount of sleep you need depends on circumstances. For example, you may need more sleep when you have lost sleep in previous days, or when your immune system needs to battle an infection, or when you are going to be taking on a particularly difficult or safety-sensitive task the next day. The science is clearly telling us that it's not so simple as a single number.

JB: So why does this seven hour number stick?

HVD: Researchers have found that if you look at people who get sick with cancer, diabetes, heart disease and then work back and ask how much sleep they got, you find that the people who survived the longest and had the fewest health consequences are the ones who sleep seven hours on average. But you've got to be very careful about how you interpret that. If you sleep much less than you need, we have good reason to believe that will impair your health. If you sleep five hours a night, and you're a person who needs seven or eight hours, we know that it increases your risk of dying sooner.

The implication of the Wall Street Journal article is that if you sleep nine hours a day, you might have bad health consequences. It's not so simple. What happens is that people who have bad health sleep more. So if you start working backward from people who have health problems, they were already sick and they slept more because their immune systems were trying to fix the problem.

JB: Isn't one of the other big, unanswered questions in sleep science: what do we need sleep for?

HVD: That's the big question we haven't answered. It's complicated. We know sleep does a lot of things for the brain and body. We know that if you get enough sleep, that the health of your immune system, your bones, your digestive system is all in better shape than if you don't sleep enough. But we don't know what sleep does to fix those problems. If you don't get enough, you have a harder time remembering things, focusing, reacting fast. We know that if you get more sleep, that helps, though you sometimes need a lot more sleep than you think to fix those problems. We just don't know what sleep is doing when we're asleep.


JB: What do you think people need to understand about sleep?

HVD: Many people think they can get by with less sleep than they need because they got used to it or found a workaround. But it turns out if you measure it very carefully—what people can do, whether they can focus very well—one of the consequences of sleep loss is that the brain becomes unstable. The brain of the person who doesn't get enough sleep most of the time can actually do most of the things it needs to do most of the time. But when it doesn't do a good job, that comes at random moments. It's precisely those moments that cause people to get into trouble.

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I don't know why we need sleep, but I believe that we do.

Humans seem to need less sleep than other mammals, most notably CATS.

Cats have the good life. Sleep 20 hours a day? Yes please!