Sleep found to repair and reorganize the brain

Most of us do it every night but we don't know why. If you miss too many nights, it might kill you.

We know why we eat, drink, breathe, and move around, but no one can explain why we need to sleep. What does seven or eight hours of snoozing really do for us? Van Savage at the Harvard Medical School and Geoffrey West of the Santa Fe Institute in New Mexico believe they have found a good answer.

One favorite explanation is that sleep is for resting the body. But as Steven Strogatz, a mathematician at Cornell University wisely points out, lying still for eight hours is no substitute for this strange state in which we spend decades of our lives "immobilized, unconscious, and vulnerable."

All mammals, from elephants to mice to humans, sleep, but in very different ways. Elephants get along on three to four hours, less than half of what humans require to stay healthy. Mice sleep about 14 hours a day. Whales keep one side of their brain awake while they sleep, allowing 1.5 hours of sleep per brain-side per day. The reason for sleep, then, must be tied to size.

It is also tied to metabolic rate, or the rate that energy is produced from food and enables animals to grow, fight, run, and reproduce. A woman is about 4,000 times the size of a mouse and has a metabolic rate that is 200 times larger. How exactly does this impact the need to sleep?

Surprisingly, no mathematical theory had ever been developed to explain how body size and metabolic rate are related to sleep. Such a theory would go a long way toward explaining the usefulness of sleep. So Savage and West constructed such a theory.

Their work yields predictions that correspond to different explanations of why we sleep. For example, if we sleep to rest our bodies, this would produce a different range of sizes than if sleep is needed only at the brain level.

Savage and West analyzed data on 96 different mammals, from the smallest to largest, shrews to whales. "We were able to distinguish between processes acting at brain level versus those on a whole-body level by using the fact that the brain does not increase in size as fast as the body. A horse is 10,000 times larger than a mouse but has a brain only about 1,000 times larger."

The fact that sleep times shorten rather than increase with body size, "is striking," Savage notes. "It is in distinct contrast to virtually all other biological rates and times, such as life span, gestation, and development of the young." For example, some whales reportedly live as long as 210 years, compared with humans who usually live 80-90 years, exceptionally long for our size. Mice have a life span of only about 2 years. Such a life span-to-size connection is likely tied to the function of sleep.

"Our data are consistent with the idea that sleep is primarily devoted to the critical activities of repair and reorganization in the brain, not the whole body, and that this reorganization probably includes learning and memory," says Savage. In the Jan. 16 issue of the Proceedings of the National Academy of Sciences, he and West report, "This leads to the conclusion that other organs and tissues do not require an analogous state because they can be repaired or reorganized during waking or resting periods."