The evolution of sleep
30th September 2008 – 2.56 pmFeeling 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.
3 Responses to “The evolution of sleep”
One could look at the spread of humans up past the arctic circle, and if the amount of time spent asleep varies with the seasons or if the optimum amount of sleep stays fixed throughout the year.
I imagine it would depend somewhat on migration patterns to find when humans started inhabiting different lattitudes, because of the timescales involved with evolution.
Studying species that have evolved to live in unchanging conditions, like the cold, dark depths of an ocean, would be interesting.
There is, of course, the issue of what came first: the physiological processes requiring us to sleep; or because we sleep our bodies use that time for the advantageous physiological process. My idea above is based on the latter of the two.
However, dolphins don't sleep as we do and instead shut down each half of their brain at different times for short periods. But this shows that even if dolphins don't sleep as such their brains still require on a regular basis the same kind of physiological processes that we experience during sleep. A similar situation is true of sharks. This gives credence to the idea that the physiological processes of sleep evolved distinctly and the pattern of sleep followed.
Whilst an extended sleep pattern did not evolve in some creatures it seems that higher-order brains require the physiological processes that occur during sleep.
We are also likely to have evolved from highly simple organisms, ones that need no sleep or complex resting conditions. It is likely that our evolving sensory systems first adapted to daylight conditions before a complex brain was formed. This would suggest that a lack of daylight conditions would force such simple creatures to rest during the night, and the physiological processes followed afterwards.
Either way, I think a lack of daylight would still be a good evolutionary explanation of why humans sleep for many hours each day. If a higher-order brain requires sleep it still makes sense for this to occur during times when we can do nothing else, and that other creatures can require the same function but perform it in a significantly different fashion shows that a lack of light for diurnal creatures could easily be a good enough reason. And if we evolved to sleep because our sensory organs developed to process light before we needed complex sleep patterns then night time may not be a direct cause of sleep but still the reason behind it.