Sleeping takes up to a third of our time. Newborn babies spend about twice that. It is inordinately difficult to remain awake for more than a full day-night cycle, without artificial substances. In humans, continuous wakefulness of the nervous system results in mental derangement; rats deprived of sleep for 5 of more days die. All mammals sleep, reptiles and birds sleep, and voluntary breathers like dolphins sleep with one brain hemisphere dormant at a time. The evolutionary trend is clear, but the function of sleep is not.

The reason of sleep suggests a deep importance. There is no universally agreed-upon answer, but there are at least three popular (and nonexclusive) guesses:

• Sleep is restorative, saving and replenishing the body’s energy stores. However, the high neural activity during sleep suggests there is more to the story;
• Sleep allows the brain to run simulations of fighting, problem solving, and other key actions before testing them out in the real world;
• Sleep plays a critical role in learning and consolidating memories and in forgetting inconsequential details. In other words, sleep allows the brain to store away the important stuff and take out the neural trash.

Recently, the spotlight has focused on REM sleep as the most important phase for locking memories into long-term encoding. In one study, rats were trained to scurry around a track for a food-reward. The researchers recorded activity in the neurons known as place-cells, which showed distinct patterns of activity depending upon the rats’ location on the track, as if a map was made by the neurons.
Later, while the rats dropped off into REM sleep, the recordings continued. During this sleep, the rats’ place-cells often repeated the exact same pattern of activity that was seen when the animals ran. The correlation was so close, the researchers claimed, that as the animal “dreamed”, they could reconstruct where it would be on the track if it had been awake—and whether the animal was dreaming of running or standing still.

The emerging idea is that information replayed during sleep might determine which events we remember later. Sleep, in this view, is akin to an off-line practice session. In several recent experiments, human subjects performing difficult tasks improved their scores between sessions on consecutive days, but not between sessions on the same day, implicating sleep is a learning process.