Sleep accounts for one-quarter to one-third of the human lifespan. But what exactly happens when you sleep? Before the 1950s, most people believed sleep was a passive activity during which the body and brain were dormant. But it turns out that sleep is a period during which the brain is engaged in a number of activities necessary to life—which are closely linked to quality of life. Learn about the science behind sleep and why sleep is essential through this informative documentary.