Feeling awfully tired recently, with one thing or another, has made me ponder the utility of sleep. It is quite a curious activity, although 'activity' may be the wrong word. I know we are gaining a better understanding of the brain and how it functions, but I also know that there is far more that we don't understand about the brain. There are also good physiological reasons for sleeping, allowing the body and mind to rest and recover, and that eschewing sleep can lead to physical and mental health problems. Even so, there is no good answer to the question of why we sleep.
This got me to thinking about how sleep came about from an evolutionary point of view. After all, it surely cannot be an accident that needing to be unconscious for many hours each day is a natural part of our schedule. People who could be active for many more hours in a day would surely have a genetic advantage over others who would be hidden away sleeping. So there must be some advantage to sleeping, particularly when we will end up spending a quarter to a third of our lives asleep.
It only took a little bit of thought for an answer to become obvious: we sleep because it gets dark. Whilst we can function perfectly well during daylight hours we cannot see well in the dark and thus are quite limited in what we can achieve. For creatures such as humans there is almost no benefit in remaining awake and alert during night times. Before the advent of reliable, continuous lighting, which occurred in less than the blink of an eye on an evolutionary timetable, there was little else to do but remain still and rest, so it is understandable that a sleep state evolved.
The physiological and mental benefits may have followed, making use of the time asleep, but that it gets dark seems like a good evolutionary answer to the question of why we sleep.