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 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.