PLoS Biology has a cozy essay entitled “Is Sleep Essential?” that addresses the mystery of the purpose of sleep.
The article looks at sleep across the whole of the animal kingdom to examine how different species sleep and whether there are any animals that don’t sleep at all.
There are no convincing cases of sleepless animals it seems, and the authors, neuroscientists Chiara Cirelli and Giulio Tononi, argue that sleep is therefore likely to be an essential function of living creatures.
The three corollaries of the null hypothesis ['sleep is not required'] do not seem to square well with the available evidence: there is no convincing case of a species that does not sleep, no clear instance of an animal that forgoes sleep without some compensatory mechanism, and no indication that one can truly go without sleep without paying a high price. What many concluded long ago still seems to hold: the case is strong for sleep serving one or more essential functions. But which ones?
The article goes on to examine the hypotheses that sleep is important for regulating the body’s core functions, the brain, individual cells and that it is common to all species and must involve something that cannot be provided by quiet wakefulness.
More interesting is the question of whether all animals dream – and perhaps most intriguing, if so, how they might dream.
Indeed, it would be interesting to discover whether dreaming is a necessary function of sleep, or whether it is specifically linked to certain neurocognitive processes or even particular creatures.