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

Universal Images Group
Universal Images Group

Seven hours is the new eight hours when it comes to sleep. According to a Wall Street Journal article published yesterday, people who live the longest in the best health get seven hours. One expert even made this frightening statement: "Eight hours or more has consistently been shown to be hazardous."

Whether you were vindicated or frightened by the news, there's more to the story. Vox called Dr. Hans Van Dongen, a leading sleep scientist based at Washington State University, to find out about optimal sleep, whether sleep apps have any positive health effects, and why you might skip running in the morning if you were up late. Here's the transcript of the interview, lightly edited for clarity.

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.

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A Fitbit by Moment Editorial/Getty Images

JB: Sleep-tracking devices are very popular now. Is there any evidence that these are good for our health?

HVD: A lot of very good technology is being used to develop these devices. But the big issue is, first of all: What do people like to get out of these devices, and secondly do they work. There's a variety of sleep tracking devices out there and they can give you a pretty reasonable estimate of how much sleep you have been getting. But they are not so great  about determining the quality of your sleep. In part, that's because quality of sleep is very difficult to measure even in labs with full technology available.

JB: Why is it so hard to measure quality of sleep?

HVD: We don't really know what the quality of sleep means. We often judge by what people say about how they slept. 'I had a restful night or not.' But if we look into what sleep looks like in terms of brain waves or blood flow, or in terms of performance consequences the next day—are you capable of doing work or not—it's very difficult to pinpoint the quality of sleep. We've never been able to say what it is that makes sleep qualitatively good or bad. So if you don't know what it is, you can't really measure it.

JB: Are there any places where you think sleep-tracking devices can be useful?

HVD: What we do know is that the amount of sleep you get, regardless of the quality, is an important driver of how well you perform the next day. If you got less sleep than you usually do, chances are high you'll have more problems being functionally optimal the next day. So the tools, the tracking devices, that have been invented are useful in the sense that they make people aware of the sleep they got and sleep they did not get. If they start to recognize that has implications for how they do the next day, people become more conscious and aware of the importance of sleep. Maybe they start to value sleep more, and get more sleep.

JB: A practical question many struggle with: is it better to get less sleep but wake up to exercise or to just sleep in?

HVD: There are three things we need to take care of for our health: sleep, exercise, and good food. But often it's a busy day. So what should you cut? The benefits of the exercise for your health are undone by not getting enough sleep so you may shoot yourself in the foot. My recommendation would be to be careful in making those decisions. Find some kind of compromise.

JB: As a sleep scientist, what do you do to get better sleep?

HVD: I have the luxury that I work in a research lab that I run. It means I can determine the work schedules here. I know one of the big things we don't do very well as a society is to work with people's individual differences. Some people are evening types, like owls. Some people are like larks. So in my lab, I like to allow for people to be able to work at the times they're most productive. So I sleep about eight hours a day and I try to make sure my sleep schedule matches my biological clock. I'm more of a night person, so I come in to work later and then stay later. That works well for me.

JB: Eight hours? You must be a nice person to work for.

HVD: People often think if you don't get enough sleep that it makes you groggy, a bad coworker. There's some truth to that but it's more complicated. It's not that sleep loss makes you more groggy but that it makes it more difficult to regulate your emotions. We have to regulate our emotions all the time, and without sleep, that becomes very challenging.

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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