In this essay I will contend that ultimately we cannot yet directly answer the question of ‘Why we sleep’ but will suggest theories that go some way in helping us understand the function of sleep. I will address the potential for future understanding created by fully understanding the key differences between non rapid eye movement sleep (NREM) and rapid eye movement sleep (REM) and suggest that our need for these two distinct states must an indication of one of the key roles of sleep. Other considerations to be examined are that sleep has been shown to replenish and saves energy, render us inactive at opportune times, help restore cognitive functions, revers changes in the waking brain and possibly help consolidate memory. The research can be approached by looking at the effects caused by lack of sleep and also by extrapolating meanings from the sleep patterns of a wide variety of different organisms.
Sleep, defined as ‘a state of immobility with greatly reduced responsiveness...(which) can be distinguished from a coma...by its rapid reversibility’ (Siegel, 2005) was divided into five distinct stages by Aserinsky and Kleiman based on patterns they found in the speed and type of brain waves that we produced. The first four stages, categorised as slow-wave sleep is defined by a slow beating brain wave much slower in type to those found whilst we are awake. In this state our muscles are not paralysed and but our brain rythums move us towards deep sleep. The fifth stage, rapid eye movement sleep, (REM) is identified by wave activity that closely resembles being awake. (Aserinsky, 1953).
One of the crucial neuronal differences between these two states is the pattern of firing activity. During the first four stages neurons fire synchronously, with a low frequency and generating a high voltage (Siegel 2003 quote). Sleep-on neurons in the forebrain are the only group of brain cells that have been found to be maximally active at this time showing the brain to be generally at rest.
Conversely our brain state during the fifth stage, is similar to being awake with similar neuronal activity across the brainstem. Given this similarity we can only infer that some aspect of the change in state must be crucial and provide key answers to why we need to sleep or at least to why we need this type of sleep. We cycle through the five stages of sleep each night with each cycle lasting approximately 90mins and although the amount of time dedicated to REM sleep grows with each cycle it cannot be said that it is the main priority.
The logical extension must be that both types of sleep provide us with something that we need. Evolutionary theories have hypothesised that sleep may be an adaptive survival function. Various interpretations of this are possible and can linked to maintaining a prone state and also conserving energy. One interesting aspect is that whilst in NREM state our motor functions are not overly limited (this is when sleepwalking may occur) but in REM state we are put into sleep paralysis which allows us to have vivid dreams and without the corresponding motor activation. The brain stops releasing neurotransmitters that would normally activate our motorneurons and also proactively releases other neurotransmitters that actively shut them down. (Siegel J. , 2003). In experiments on rats, scientists found that GABA and glycine caused REM sleep paralysis by ‘switching off’