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Though it is extraordinarily commonplace, there is much about sleep that remains a mystery. And, while we all spend one-third of our lives doing it, there is still no universal agreement regarding the singular purpose or function of sleep. Only in the last few decades have we even begun to unravel sleep's true secrets. However, there are at least three common theories as to why we sleep, but it is unknown which (if any) are in fact correct.
The restorative theory of sleep is the most accepted explanation for why we sleep. It suggests that sleep restores tissue and prepares our bodies for the next day. This may involve clearing accumulated neurotransmitters from our brain as well as other tissue repairs that occur throughout our bodies. More specifically, the glymphatic system flushes chemicals from the brain during sleep, including adenosine, the substance largely responsible for increasing levels of sleepiness during wakefulness.
This alternative explanation suggests that sleep increases our ability to survive. As nighttime can be dangerous—especially in animals at risk from predators—it makes sense to seek a safe refuge. By avoiding dangers, the animal lives longer and is more likely to reproduce. Thus, sleep becomes an adaptive advantage. When morning light returns, it is a powerful stimulus for wakefulness, further preserving the evolutionary advantage of responding to the natural day-night cycles.
Others theorize that sleep is a means to conserve energy. In a sense, by sleeping we are able to spend part of our time functioning at a lower metabolism. Thus, our overall caloric needs are reduced. If that time were spent awake, we may not have enough food to survive. It also allows time to create glycogen, an energy store that is used as the brain's fuel reserve. Though metabolism slows during slow-wave sleep, the brain is extremely active during rapid eye movement (REM) sleep, therefore not fully accounting for the conservation.
Despite it being a phenomenon that we may not fully comprehend, sleep is critical to our daily health. Not only is it refreshing, but sleep also helps with problem-solving, finding solutions and making connections, strengthening immunity, reducing the risk of infection, growing and developing, learning, and forming memories. No wonder we all pine for that perfect night of it!