There’s been a ton of research done on the subject, but nobody’s been able to definitively nail down why we need sleep. Since our bodies crave sleep like they crave food and air, we can conclude sleep is just as critical to our survival as calories and oxygen, and the deleterious effects of insomnia support this conclusion. There are quite a few theories to explain why sleep is so important. Here are three of them. The energy conservation theory suggests during our evolution, we adapted to lose consciousness over a period of hours in order to conserve energy, helping us effectively utilize the calories we got from food, which was much less plentiful for our prehistoric ancestors. During sleep, our metabolism slows by as much as ten percent, as both our core temperature and caloric demands decrease, helping reduce our need for food. The restorative theory posits sleep gives both the brain and body the opportunity to restore themselves after the stresses of the day. There’s a lot of evidence to support this theory, including the fact that total sleep deprivation will eliminate immune function and lead to death in animals in a few weeks, restorative hormones are released mostly or only during sleep, and the body seems to clear itself of a harmful cell byproduct called adenosine while we sleep. The brain plasticity theory focuses on learning and development. The brain is an adaptive organ, and new research suggests sleep helps it develop and maintain its plasticity. The link between sleep and brain plasticity in both adults and children has become clearer in recent years, as evidenced by the fact that sleep deprivation has detrimental effects on both children’s brain development and adults’ cognitive function.