Why We Sleep

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Maybe you have a big report due first thing in the morning. Or you're trying to deliver a truckload of fish before the wholesale market opens 150 miles away. Whatever the reason, you decide to stifle that yawn and push through the night. Sure, you've been awake 16 hours, but you have a giant thermos of coffee and a few tunes to keep you going. Your body, of course, is fighting you every step of the way.

Whether or not you realize it, your brain has already started to check out for the night.

That yawn was the first sign that you're not so awake as you think.

After about 18 hours without sleep, your reaction time begins to slow from a quarter of a second to half a second and then longer. If you're like most people, you will start to experience bouts of microsleep—moments when you zone out for anywhere from two to 20 seconds and drift out of your lane or find that you have to keep rereading the same passage. Your eyelids start to droop more severely, and by the 20-hour mark you 404 Not Found

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begin to nod off. Your reaction time, studies show, is roughly the same as someone who has a blood-alcohol level of 0.08—high enough to get you arrested for driving under the influence in 49 states. You forget to do things like double-check the spelling of a name or set the brake when you stop on a hill.

Although you may get a second wind with the rising of the sun, the longer you stay up, the more your condition deteriorates. "By the second night, oh, my goodness, it's extremely dramatic—beyond double what it was the first night," says David Dinges, a sleep expert at the University of Pennsylvania School of Medicine. "You fall massively off the cliff."

You don't need to pull an all-nighter, work 24-hour shifts or hold down a couple of jobs to know that at some point you just have to crash. All through the animal kingdom, sleep ranks right up there with food, water and sexual intercourse for the survival of the species. Everybody does it, from fruit flies to Homo sapiens. Yet despite its clear necessity and lots of investigation, scientists still don't know precisely what sleep is for.

Is it to refresh the body? Not really. Researchers have yet to find any vital biological function that sleep restores. As far as anyone can tell, muscles don't need sleep, just intermittent periods of relaxation. The rest of the body chugs along seemingly unaware of whether the brain is asleep or awake.

Is it to refresh the mind? That's closer to the mark. The brain benefits from a good night's sleep. But there is no agreement among sleep researchers about what form that benefit takes. One theory is that sleep allows the brain to review and consolidate all the streams of information it gathered while awake. Another suggests that we sleep in order to allow the brain to stock up on fuel and flush out wastes. A third, which has been gaining currency, is that sleep operates in some mysterious way to help you master various skills, such as how to play the piano and ride a bike.

Most of the new science of sleep has emerged quite recently, as researchers supplement EEGs—the old-fashioned electroencephalograms that are a recording of the waves of electrical activity in the brain—with far more sophisticated imaging and neurological mapping techniques. With the new equipment, scientists are able to take increasingly detailed pictures of the sleeping brain, observing precisely what it is doing while it rests, down to the individual neuron. "In the past year or two, everything seemed to click together," says Dr. Giulio Tononi, a neurobiologist and psychiatrist at the University of Wisconsin at Madison. "Suddenly we have hypotheses that could explain lots of things. Whether they're right is a different story. But I feel different from a few years ago, when the thinking was, 'Who knows? Sleep could be anything.'"

Those Shifty Eyes
Without a good theory of what sleep is for, scientists for many years concentrated on describing what it is—and treating conditions that interfere with it, such as anxiety, restless-leg syndrome and sleep apnea. They've learned that most mammals, with the possible exception of dolphins and whales, cycle between two distinct phases of sleep, one of which is characterized by rapid eye movement—the famous REM sleep. The other is called, straightforwardly enough, non-REM sleep. Humans generally take about 90 minutes to complete a full cycle of REM and non-REM sleep. As dawn approaches, however, we spend more and more of that time in REM sleep and less in non-REM sleep.

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