Sleep, Evolution and Brains.

Much research points to important and diverse effects of sleep on neural and cognitive functioning, suggesting that the relationship between sleep and the brain holds the key [Hobson, 2005]. Sleep is associated with distinctive patterns of neural activity which differ between rapid-eye movement (REM) sleep and non-REM sleep [Staunton, 2005]. Sleep is thought to play a role in synaptic remodelling [Jha et al., 2005], and sleep deprivation negatively impacts on a variety of aspects of cognitive function, including learning, reaction times and attention [Hobson, 2005; Staunton, 2005]. The biology and functions of sleep have mostly been tackled using experiments. By carefully manipulating certain variables and holding others constant, researchers have attempted to isolate effects that are specific to sleep and to particular aspects of sleep (notably REM vs. nonREM). Complementing experimental approaches are cross-species comparisons, which potentially provide important evidence about biological function and evolution. It has been known for many years that the amount of time spent sleeping is highly variable across mammal species, from about 3 h to about 20 h per day [Siegel, 2009]. In principle, any hypothesis for the function of sleep should be capable of explaining this variation, but pinWhy do we sleep? In a sense, it is equally valid to ask why we wake, except that the functions of the things we do while awake – eating and reproducing, for example – are quite obvious, whereas the same is not true for the things that we do while asleep – such as rapid eye movements and dreaming. This is why the function of sleep has exercised the minds of scientists, writers and philosophers since Aristotle. Sleep appears to be biologically necessary: sleep deprivation in mammals is followed by a tiredness-induced ‘rebound’ and, if chronic, leads to impaired physiological and mental functioning, or, in the extreme, even death [Allada and Siegel, 2008]. Yet despite its apparent necessity, a consensus on the function of sleep remains elusive: prominent hypotheses include conservation of energy when activities are unprofitable or impossible [Siegel, 2009], memory consolidation [Stickgold and Walker, 2007] or some physiological function, such as maintenance of the immune system [Opp and Krueger, 2015]. So many different hypotheses have been proposed that some have even suggested that sleep has no specific biological function [Rial et al., 2007], although, as Rechtschaffen [1971] asserted, in that case sleep must be ‘the biggest mistake the evolutionary process ever made’. Published online: February 12, 2016