Sleep is something every vertebrate does. Why they do this is unknown - although it does seem to enhance learning and clear a build-up of nasty metabolites - but what is known is that it can vary considerably from species to species. Peculiarly, humans (who clock in ~8 hours sleep) seem to nap less than other mammals (who can push upwards of 20 hours of sleep). New research may have identified why we evolved to sleep less. And crucially, this also sheds light on why we sleep in the first place
This new study examined dozens of different species and found there was a correlation between the density of neurons in a brain and the length of time the animal had to sleep. But not in the direction you might expect. A higher neuronal density resulted in less sleep being needed by the animal. Humans have one of the highest numbers of neurons found in any mammal; explaining why we need relatively little sleep.
However, it is a tad more complicated than that. The overall number of neurons also increased sleep requirements. So humans evolved big brains with lots of neurons packed in. The increasing neuronal density meant we needed to sleep less, but the increasing number of neurons meant we needed to sleep more (although they don't quite balance each other out). That's why we still need some sleep.
But why does the neuronal density influence sleeping length in the first place? It seems a tad random.
In fact, this correlation isn't random. It was actually predicted by the scientist who carried out this research. They were able to do this based on the metabolite hypothesis.
Basically, bi-products of metabolism (called metabolites) build up in the brain during the day. Sleep increases the flow of cerebrospinal fluid (CSF) into the brain, washing away these metabolites before they can build up to toxic levels. During the day CSF only cleans out the surface of the brain. The scientist behind this latest research predicted that since animals with higher number of neurons would have more crammed into the surface, more neurons would be "cleaned" during the day allowing the animals to go longer without sleeping.
If you're having trouble picturing that; don't worry. I made a handy picture so you don't have to!
And that's the big reason this research is so significant. It confirms the authors' predictions about how neuronal density should impact sleeping requirements. This provides further evidence in support of the metabolic hypothesis from which they derived those predictions.
Perhaps we finally are honing in on why people do something as silly as shut themselves off from the world for hours a day. And then they go to sleep.
Reference
Herculano-Houzel, S. (2015, October). Decreasing sleep requirement with increasing numbers of neurons as a driver for bigger brains and bodies in mammalian evolution. In Proc. R. Soc. B (Vol. 282, No. 1816, p. 20151853). The Royal Society.
