Why do we sleep?

We may not always think about the reason we sleep, but most of us know, that at some level, sleep makes us feel better. We are more alert, more energetic, happier, and better able to complete tasks, if we have a good night of sleep. The fact that sleep makes us feel better and lack of it makes us feel worse only begins to explain why sleep might be necessary.

A way to think about sleep is to compare it to eating. Hunger is a protective mechanism that ensures we eat the nutrients our bodies need to grow, repair itself, and function properly. Although it may be pretty easy to see why we eat, sleeping is not much different.

Sleeping and eating are both regulated by internal drives. Going without food makes you feel hungry. The longer you go without food, the worse your hunger grows. And just as eating relieves hunger, sleeping relieves sleepiness. Sleeping ensures we get the sleep we need. Why do we need sleep at all? There are many theories being tested to help understand this question.

One of the earliest theories of sleep suggests that inactivity at night is an adaptation that served a survival function by keeping organisms out of harm’s way at times when they would be vulnerable. The theory suggests that animals that were able to stay still and quiet during these periods of vulnerability had an advantage over other animals that remained active. These animals did not have accidents during activities in the dark, and were not killed by predators. Through natural selection, the behavioral strategy presumably evolved to become what we now recognize as sleep.

A simple counter-argument to this theory is that it is always safer to remain conscious in order to be able to react to an emergency, even if lying still in the dark at night; therefore, there does not seem to be any advantage of being unconscious and asleep if safety is the most important.

Is it to refresh your body? Researchers have yet to find any vital biological function that sleep restores. As far as anyone can tell, muscles don’t need sleep, just periods of relaxation. The rest of your body goes along with really “knowing” whether the brain is asleep or not.

Is it to refresh the mind? The brain benefits from a good night’s sleep, but brain researchers do not agree as to what kind of benefit sleeping gives your brain. One theory is that sleep allows the brain to review and compact the vast amount of information it gathered while awake. Another suggests that we sleep in order to allow the brain to stock up on fuel and flush out wastes. A third is that sleep operates in some mysterious way to help you master various skills, such as how to play the piano and ride a bike.

Although these theories remain theories, science has made advances in discovering what happens during sleep and what controls the cycles of sleep and wakefulness that help define our lives. While this research does not directly answer the question, “Why do we sleep?” it does set the stage for putting that question in a new context and generating new knowledge about this essential part of life.