Why do we sleep?

By: Charles W. Bryant

Getting More Sleep Image Gallery The old joke is that we sleep to cure sleepiness. See more sleep pictures.
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Most adults who aren't insomniacs love it when that time of the night rolls around. At last, they can leave the worries of the day behind and fall into a lovely, relaxing sleep. A night of sawing logs can leave you feeling refreshed and ready for the coming day. Some people even make better decisions after "sleeping on it" for the night. In fact, an easier article to write might be "why do we love to sleep?." The trouble is, after all these years and millions of hours of research, science still isn't exactly sure why we sleep. There's an old joke among sleep scientists that the purpose of sleep is to cure sleepiness. But humor aside, there are a few theories on the matter.

One theory is that sleep is our brain's opportunity to file away or discard memories, thoughts and events from the previous day. Think of your brain as a desktop computer, and sleep as your way of organizing everything after a long day of work. Some of the files you may want to store away as permanent memory, but others you might want to put directly into the recycling bin. A study performed on lab rats found the same neurons that fired when a rat was completing a task (like working its way through a maze) also fired when the rat was sleeping [source: Gorman]. This indicates that a review process may take place while we sleep. (However, it was also learned that the same neurons were reactivated during periods of rest.)


The filing theory jibes with the fact that we often wake up with a clearer head. Have you ever decided to "sleep on it" before you made a tough decision? There might be something to that strategy. It's been theorized that, during the night, some of the unnecessary emotional responses and extra information that clouded your thoughts are put into the recycling bin, leaving only the information necessary to make the decision. The next morning you awake with a crystal clear perspective on something that had previously seemed muddled.

But that's not the only theory about the function of sleep. Find out more on the next page.


Function of Sleep

You may think that sleep's function is to give your body a break, but you'd be wrong. Our muscles, and body in general, do need rest, but actual sleep doesn't contribute anything to the process. The body only knows when it's at rest; it doesn't care whether you're sleeping or just relaxing in your recliner. But that doesn't mean that your brain doesn't need rest as well. That's why many researchers believe that sleep is just that -- a chance for parts of your brain to take a break. It's an opportunity for your neural connections to strengthen and recharge. However, even if there's some truth to this, science hasn't pinpointed the exact reason why sleep facilitates it. Some scientists feel sleep research has been overanalyzed, and that researchers are simply unwilling to accept the notion our brains need a break.

For example, the head of sleep research at the Walter Reed Army Institute of Research posits that the brain simply gets tired. He backs up his theory with data that shows the brain experiences a dramatic drop-off in performance after it's been awake for more than 24 hours. The brain runs on glucose, and test results show that even when there's plenty of glucose available, after the brain has been awake for a full day, it simply doesn't use it. After the 24 hour window, the use of glucose stabilizes, but the brain's performance continues to suffer. No one is sure why this happens, but the idea that the brain runs on a cycle is gaining credence. It appears the brain literally needs sleep in order to refuel.


We're not there yet, but we're closer than ever to uncovering the mysteries of what exactly happens when we sleep. Perhaps one day science won't have to fall back on its old joke: "We need sleep to cure sleepiness."



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  • Braun, David. "Why do we Sleep? Scientists are Still Trying to Find Out." National Geographic. 2010. (Aug.13, 2010)http://blogs.nationalgeographic.com/blogs/news/chiefeditor/2009/08/why-we-sleep-is-a-mystery.html
  • Gorman, Christine. "Why We Sleep." Time. Dec. 17, 2004. (Aug. 13, 2010)http://www.time.com/time/magazine/article/0,9171,1009765-1,00.html
  • Schaffer, Amanda. "Why Do We Sleep?" Slate. March 26, 2007. (Aug. 13, 2010) http://www.slate.com/id/2162475/entry/2162476/
  • "Why Do We Sleep, Anyway?" Harvard. 2010. (Aug. 13, 2010) http://healthysleep.med.harvard.edu/healthy/matters/benefits-of-sleep/why-do-we-sleep