It is such an elemental question, yet it is one that many people never ask and most people don’t often think about. There are three main schools of thought regarding why we sleep. So why do we sleep?
The first explanation is that sleep is restorative, that all animals need to sleep for some vital biological process to occur. This explanation seems obvious, there is an almost unthinking consensus that this is the main function of sleep. You only have to go a few days without sleep for this explanation to appear totally correct. However, the problem is that there is no scientific agreement on what exact biological process is occurring when we sleep. No one really understands why sleep seems so good for us or why sleep deprivation is so bad for us.
Another explanation is that sleep fulfils a role in cognitive development, providing a time and space for the brain to process the day’s activities and to store memories and information. Dreams can be seen as part of this process of assessment and storage.
A new explanation for why we sleep is that it fulfils an evolutionary function. Put simply, this theory proposes that sleep ensures that animals are only active when the benefits of moving, such as getting food and mates, offset the costs, like risk of accident and the burning calories. In other words, sleeping when it is dark protects us from hurting ourselves or wasting our energy.
While a definitive answer for why we do sleep is, for now, out of reach, all three are persuasive in their own right and the truth is probably that sleep fulfils all three functions.