How much sleep is enough?

Everyone has an opinion on how much sleep they need. I need around eight hours of sleep per night to feel energetic and cheerful. Others claim they function well on as little as six hours. Are our sleep needs really that different? Maybe people think they don’t need much sleep, while in reality their performance suffers. Rather than continuing to speculate, I set out to collect some basic facts on sleep.

I asked simple questions. Why, biologically speaking, do we sleep at all? Do some people need more sleep than others? What happens when you don’t sleep enough? And how much sleep do experts recommend we get, anyway?

It turns out that only the last of those questions has a clear answer. The United States National Sleep Foundation recommends that adults between the ages of 26 and 64 sleep seven to nine hours per night, and that adults over 65 sleep seven to eight hours per night. For adults, they write, a sleep duration between six and seven hours or between nine and ten “may be appropriate”. Kids and young adults need substantially more sleep. And so do professional athletes. Isn’t that a clue that being a top performer requires plenty of sleep?

Beyond recommendations for sleep duration, though, the science of sleep is in its infancy. For instance, scientists aren’t sure what sleep is for. Some researchers have recently shown that a mouse’s brain cleans itself during sleep, removing waste products. That could be the case in humans too. Sleep’s other possible functions include improving wound healing and strengthening certain neural connections to essentially organize information. But we just don’t know for sure. In 2010, William Dement, a prominent sleep researcher, summarized the state of research on the purpose of sleep as follows:

As far as I know, the only reason we need to sleep that is really, really solid is because we get sleepy. (Via National Geographic.)

I think we can all agree that’s a good reason to sleep.

Aside from feeling sleepy, not getting enough sleep can have serious consequences. Even mild sleep deprivation, say sleeping a few hours less per night than is optimal, can affect your judgment or reaction time. Chronic sleep deprivation could increase your chance of developing diabetes and can increase your stress level, according to Russell Foster, a British sleep researcher. He emphasizes that “there’s a whole raft of things associated with sleep loss that are more than just a mildly impaired brain, which is where … most people think that sleep loss resides”.

So how can you determine whether you’re getting “enough” sleep? Start by looking for telltale signs: do you always need an alarm clock to get up? Do you yawn throughout the day? Do you fall asleep at work or—God forbid—while driving? (If falling asleep while driving sounds dangerous, it is! Unfortunately, 1 in 25 U.S. drivers reported having fallen asleep at the wheel in the previous 30 days and “drowsy driving” caused 72,000 car crashes in the United States in 2013 alone!) If you need coffee throughout the day to stay alert, that’s another sign you’re not sleeping enough.

I am lucky in that I can usually immediately tell when I didn’t sleep enough. When that happens, all of life suddenly seems more difficult than usual, and I find myself banging into furniture and omitting words while writing. But it may not be as clear to you whether you’re sleeping enough to perform optimally. How can you find out?

Try this. For two weeks, record how you feel when you wake up. Keep track of how you feel during the day, too, and how you feel in the evening. And collect objective information: are you yawning? Did you nod off during a meeting? Also measure how much you slept, perhaps using an app such as Sleep Cycle or Sleep as Android.

Then, for the next two weeks, sleep one more hour than you usually would. Again, record how you feel and collect objective information.

At the end of the four weeks, compare your notes. Did you feel better when you slept more? If you can compare your productivity in some objective way, that’s even better. Maybe you’ll find that the extra hour of sleep increases your performance, even if you thought you were performing well before. If so, rinse and repeat!

I left one question for the end. Within an age bracket, do some people need more sleep than others? The answer is yes. Some people have a gene called DEC2, which reduces their need for sleep to four to six hours per night, with no apparent side effects.

Unfortunately, odds are you do not have DEC2—scientists estimate that it is present in perhaps one percent of the population. Maybe you are one of the lucky ones, but most likely, you’ll just have to get your seven to nine hours of sleep.

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