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The Secret Life of the Brain
History of the Brain 3-D Brain Anatomy Mind Illusions Scanning the Brain The Episodes:

Episode 3: The Teenage Brain - Why Do We Sleep?


Teenager sleeping

Baby sleeping

If you try to make a list of the things you did in the last 15 years, it would probably look like this: three years in junior-high, 20 months with your boyfriend, a year of study in Italy. Forgetting anything? As a matter of fact, you are leaving out the one thing that you spent the most time doing. Sleep. This major time consumer took about five years from the past 15.

While adults spend about one third of their time sleeping, babies and toddlers sleep away half of their early childhood. It cannot be the terrible waste of time that it seems. Or can it be? Embarrassingly, scientists still cannot persuasively point out the biological function of sleep. Sex, eating, and sleeping constitute the triad of basic impulses of human beings. Yet, while the functions of the first two have been obvious for millennia, it is not clear why we crave to spend a third of our life in bed.

Over the past decade, new findings that may lead to the resolution of this conundrum have been accumulating. Many scientists believe that in a few years we will understand not just why we sleep, but also what biochemical mechanisms underlie this odd activity.

The brain during sleep may review and sort the knowledge that it has encountered during the day.

Koala sleeping
Koalas spend up to 20 hours a day sleeping.
The first few hints for the function of sleep came from observations on animals. All mammals sleep, as do birds and even bees. One theory suggests that sleep is a simple protection mechanism, a way to keep animals quiet and still, so that they attract less attention, and thus are less noticeable to predators. This stillness is particularly important when the animal is most vulnerable, which for many animals, is during the dark of night. But comparing sleep patterns of different species suggests that this may be too simplistic an explanation. Opossum, for example, sleep up to 20 hours a day. Giraffes, on the other hand, spend no more than 20 minutes a day sleeping, and don't even bother to recline their cumbersome bodies. Dolphins and whales also spend a very short time sleeping, and continue to swim even as they catch their Zs. Some scientists even claim that dolphins let only half of their brain sleep at a time. This diversity of sleeping patterns implies that sleep is more than just a way of keeping animals quiet. There must be another explanation.

Clearly, sleep is an opportunity to rest. Hence, many theorists have hypothesized that the main purpose of sleep is to enable the muscles and the brain to recuperate after a busy day. But measuring the electric activity of the brain unveils the shortcomings of this theory: A sleeping brain is far from dormant.

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