How Much Sleep Do You Really Need?

Some lucky people can get by on only four to six hours of shut-eye a night. We hate to break it to you, but you’re probably not one of them. Here, the truth about how much sleep you really need — and what can happen if you don’t get it.

Thomas Edison thought it was waste of time. Bill Clinton used to brag that he could get by on only five hours of it a night. And Margaret Thatcher once famously said that it was “for wimps.” Sleep! Who needs it, right?

Well, actually, you need it. And so does everyone else — including self-proclaimed “short sleepers” like Clinton. In a 2008 interview with CNN’s Anjali Rao, the former commander-in-chief admitted that his previous years of sleep-deprivation had taken a serious toll on both his body and his mind. “In my long political career,” he told Rao, “most of the mistakes I made, I made when I was too tired, because I tried too hard and worked too hard.”

Clinton’s situation is somewhat unique, of course — who wouldn’t stay up nights if they had the fate of the country in their hands? — but the fact is that we’re all guilty of overextending ourselves. According to the National Sleep Foundation, Americans average about 6.9 hours of sleep a night, which, though not exactly bad, isn’t good, either.

So, what is good? How much shut-eye is enough to keep us functioning through the day, the week, our lives?

What’s Your Magic Number?

Technically, there is no one-size-fits-all prescription for sleep. Everyone’s needs are different, so while some people may feel the effects of just one night of missed slumber, others may be able to endure three or four nights before slowing down. For the majority of the population, however, seven to nine hours is considered ideal — anything less than that will start to negatively affect your ability to perform.

“When sleep gets below seven hours time in bed for otherwise healthy adults, a cumulative deficit develops for the majority of people,” explains David Dinges, PhD, chief of the division of sleep and chronobiology at the University of Pennsylvania. “That is, each day, their alertness level, attention, and cognitive functions become steadily worse.”

As one of the leading researchers in the field of sleep deprivation, Dinges has seen these effects firsthand. In one landmark study from 2003, he and his colleague Hans Van Dongen assigned dozens of subjects to a two-week program of restricted sleep in a lab. Some slept four hours a night, others six, and still others eight. During the day, each group took part in psychomotor vigilance task (PVT) evaluations every two hours to test their cognitive response times. (The PVT involves sitting in front of a computer screen for 10 minutes and pressing the space bar whenever you see a predetermined visual stimulus. Pilots, truck drivers, and astronauts use it to measure their attention span and alertness.)

Not surprisingly, people assigned to the eight-hour program showed virtually no cognitive decline or lapses into sleepiness over the two-week period. People in the four- and six-hour groups, however, began to suffer almost immediately and performed progressively worse as the study went on. By day six, a quarter of those on the latter program were falling asleep at the computer. And by day 14, their level of impairment was equal to that of a group who had been deprived of sleep for 24 consecutive hours.

“The rate of deterioration is faster with four hours a night than five, faster with five than six, faster with six than seven,” Dinges says. “And the more nights you go without adequate sleep, the worse it gets. Eventually, even if you’re getting six hours a night, it will progress to a deficit equivalent to going one or two nights without any sleep” — which, cognitively speaking, is like being legally drunk.

The Dangers of Sleep Deprivation

You would never think of going to work intoxicated, but you probably don’t think twice about going to work tired — even though exhaustion may be just as damaging to your performance and your health. In fact, research shows that chronic sleep deprivation may raise your risk for heart disease, stroke, dementia, and even cancer — and those are just the long-term effects. The immediate consequences can be harmful, too.

“People with chronic sleep deficits are less alert during the day, more likely to be inattentive, and more likely to have problems concentrating or performing tasks that require them to think fast,” Dinges warns. “Their memory may also be affected, and, of course, they’re at greater risk for driving drowsy.”

This isn’t just dangerous for you — it could be deadly to others, as well. According to the National Highway Traffic Safety Administration, driver fatigue causes 56,000 motor vehicle accidents each year, 1,500 of which result in death. And it doesn’t take much to happen. A half-second cognitive delay may not seem like a big deal, but it takes only a two- to four-second lapse — what’s called microsleep — for someone to lose control of a car. You may not even notice you’re tired until it’s too late.

Under chronic sleep conditions, people may feel tired at first but then wrongly assume their bodies have adjusted to the deficit, Dinges explains. “They say, ‘I think I’ve adapted’ — but in fact they continue to get progressively worse. They’re just no longer aware that the sleep debt is affecting their performance,” he says. “We used to wonder why people couldn’t understand, but now we realize that, at some level, the brain cannot introspect or know what its state is. Once the condition is chronic, the brain just redefines what’s normal.”

One problem may be that we’re distracted from our fatigue by stimulation from work or friends or caffeine. These so-called “countermeasures” keep us awake — but often at a price. You might not feel the effects of missed sleep as quickly or as severely as, say, someone in a sleep lab, but that doesn’t mean you’re not experiencing them. In some people, the loss of function may be so gradual that they’re not aware it’s happening until they’re literally falling asleep at their desks.

“Some people just don’t deteriorate as fast as others,” Dinges says. “Many are okay on Monday or Tuesday but may start to feel worse as the week goes on. By Friday, it can be quite bad.”

The Exceptions to the Rule

Poor sleep may affect everyone at different rates, but it will eventually affect everyone — except, perhaps, the very small percentage of true “short-sleepers.”

According to scientists at Germany’s Ludwig Maximalians University of Munich, certain people may be able to operate on just four to five hours of sleep (without naps or coffee!), thanks in part to a gene called ABCC9. Experts say, however, that the gene is exceedingly rare — only three to five percent of the population actually has it, though many more (Clinton, perhaps) may think they have it. In fact, out of every 100 people who believe they can function on less than six hours a night, only about five people really can.

“Part of understanding your sleep is understanding not just how much you need at night and how to get it, but also what your vulnerability is to not getting it,” Dinges says. “If you’re constantly overemotional or drowsy, you’re not getting enough sleep — and if you think you are, there’s something wrong with your sleep.”

It’s not just how much you sleep that counts, experts say, but also how well you sleep. Anything that interferes with your body’s ability to rest can add to your sleep debt, which becomes more and more difficult to erase over time. According to the National Sleep Foundation and research from Harvard Medical School, there’s no real way to recoup all the sleep you’ve lost. But that doesn’t mean you shouldn’t try.

“Sleeping in on the weekend can actually be quite useful,” Dinges notes. “Does it completely reverse your sleep debt? Not entirely. But getting some extra hours every once in a while may help. We just can’t claim to know how much extra.

“Really, in the end,” he says, “the true solution to this is for people to prioritize their sleep at a much higher level. Because every day that you get adequate sleep, you’re capable of functioning at your best.”

Allison Takeda