Why do we sleep? It's such a basic question, but there's no agreed answer and debate continues to set out ideas.
An article in Nature Reviews Neuroscience suggests that rather than looking for a single universal evolutionary function for sleep, we should see the dozing animal as part of a wider sweep of nature which includes the dormant states of plants. These phases of resting and activity reflect nature's best efforts to make use of the resources available. So a creature which depends on hunting during the night will shape its sleep patterns to optimise its chances of success.
I've always thought that sleeping and waking are not opposites, but are integral to each other - and so looking for the purpose of sleep is the same as looking for the purpose of being awake. Sleep isn't a lesser state which needs to explain itself. We should consider the possibility that sleep is the default position, a state of dormancy being closer to nature than being awake. There are some simple creatures that spend almost all their lifetime in a state of rest, stirred into brief rushes of activity only when threatened or needing food. For them being awake is an occasional unpleasant aberration, while the majority of their life is the calm rhythm of inactivity.
In the living world, for plants as well as animals, the patterns of day and night, rising and falling, birth and death, winter and summer, are profoundly rooted - and in this pattern must also lie the idea of sleeping and waking, one not possible without the other.
But the search for an explanation for sleep shouldn't beguile us into thinking that being awake is the higher state of being. Being awake is what we prioritise because it's where our culture stores its prizes. It's where we work, promote our individualism, assert our power, it's where economic status resides. Sleep has none of this, it's a mysterious, curious place, nothing to do with money or social status. It operates on a level much closer to nature. It's where we're only a few unconscious breaths away from other living creatures.