Despite centuries of speculation and research, we still do not know what sleep really is, or exactly what it is for. Allan Rechtschaffen suggested that ‘If sleep does not serve an absolute vital function, then it is the biggest mistake the evolutionary process ever made’ (1971, p.88) and it does seem reasonable to conclude that unless it gave a species an advantage, sleep would have become extinct by now. Noting that all animals sleep, Paterson (2012) shows how the sleep of some animal species has adapted in surprising ways in order to meet to particular needs. She also discusses some of the functions that we now think sleep serves, although these modern ideas contrast with theories that have developed through the ages.
According to Scrivner (2012), in ancient Greek mythology sleep
“…was made the jurisdiction of anthropomorphic deities and weird demons whose actions determined both our shifting states of consciousness as well as the endless shift of day into night and back again. The Greek god of sleep, Hypnos, and Nyx, the goddess of the night, are chased away each morning by … Eos, the dawn” (Scrivner 2012, p.271).
However, in ancient Greek philosophy, in one of the earliest explorations or the causes and purposes of sleep Aristotle (384 BCE–322 BCE) made what now seems like a statement of the obvious:
“It is inevitable that every creature which wakes must also be capable of sleeping, since it is impossible that it should continue actualizing its powers perpetually. So, also, it is impossible for any animal to continue always sleeping.” (Aristotle)
He went on to explain how sleep relates to the cardinal humours (blood, phlegm, black bile and yellow bile) and to changes in body temperature that result from eating and digestion
“… in every animal the hot naturally tends to move […] upwards, but when it has reached the parts above [becoming cool], it turns back again, and moves downwards in a mass. This explains why fits of drowsiness are especially apt to come on after meals; […]. When, therefore, this comes to a stand it weighs a person down and causes him to nod, but when it has actually sunk downwards, and by its return has repulsed the hot, sleep comes on …” (Aristotle).
Zimmer (2005) shows how Thomas Willis, though a pioneer in neuroscience, was still a man of the mid-seventeenth century and held that sprits flowed through the nervous system; emotions were produced by movements of the sensitive soul (as opposed to the humours of the heart) and spirits, which expand in happiness, could not flow without rest – hence the need for sleep. Willis attributed narcolepsy (although it was not formally identified then) and sleepiness to an excess of blood in the brain which cramped the animal spirits, making them unable to flow; he prescribed bleeding and newly available coffee. Sleepwalking resulted from spirits travelling down the spine causing the person to walk. Although such ideas might seem quaint now, Willis, like Aristotle, was at least finding explanations for phenomena in sleep that excluded supernatural intervention.
In the nineteenth century Robert Macnish, a Scottish physician and surgeon, was referring to the phenomenon of night terrors – known then as nightmare (see Green 2012) – not as the visitation of evil spirits, but as an illusion; however, he noted that ‘Many a good ghost story has had its source in the illusions of nightmare’ (Macnish n.d., p.130; first published 1834). Evenso, while a rational scientist, Macnish still did not know what sleep was for:
“Sleep, being a natural process, takes place in general without any very apparent cause. It becomes, as it were, a habit, into which we insensibly fall at stated periods, as we fall into other natural or acquired habits” (Macnish n.d., p.17; first published 1834)
Early in the twentieth century Addington Bruce reviewed some of the theories about the function and causes of sleep. He observed that explanations had ‘usually resolved themselves into descriptions of states that accompany sleep rather than demonstrations of the factors that cause it’ (Bruce n.d., pp.2–3; first published 1915). For example, he noted that ‘deficiency of blood in the brain’ (p.3) did not signify a cause, but proved only that circulatory changes occur in sleep. Bruce also dismissed theories that sleep can be attributed to accumulation of toxins in the blood or that monotony (see Sidis 2010; first published 1909) was the cause. He asserted that sleep:
“…is an active positive, positive function, a protective instinct of gradual evolution … its object being not so much the recuperation of the organism … as to save [it] from the destructive consequences of uninterrupted activity” (pp.8–9).
He did, however, suggest that ‘most us sleep longer than is really necessary’ (p30).
While research on sleep flourished during the twentieth century, and into the twenty-first, – see Kroker (2007) for a detailed account – suggestions that we could sleep less, or barely at all, have persisted and the idea that sleep is a waste of time remains widespread. In the 1950s and 60s writers looked forward to times when we might sleep less – the most bizarre suggestion being the ‘electrosone’, a device that was to allow us to have only two hours’ sleep a night (see Green 2012a).
At the present time debate continues about unprescribed use of modafinil, a stimulant used for treatment of narcolepsy, and whether it can allow us to get by with less sleep (for example, see this Guardian article.) But whatever the short-term benefits may be, it seems unlikely that we can safely reduce our sleep substantially in the longer term.
We know that sleep is essential for memory, learning and performance of many activities (see Green 2012b, for example) and that the links between sleep and good health are many. Put one way, we might say that if the Good Lord did not intend us to sleep He would not have given us the night – or put another: millennia of evolution are unlikely to have got it so wrong.
Aristotle (2011-09-30). On Sleep and Sleeplessness [Illustrated]. Kindle Edition.
Bruce, H.A. (n.d) Sleep and Sleeplessness. Kila, MT: Kessinger Publishing, LLC. (Original work published 1915).
Green, A. (2012a) ‘Sleeping on it.’ In A. Green and A. Westcombe (eds) Sleep: Multiprofessional Perspectives. London: Jessica Kingsley Publishers.
Green, A. (2012b) ‘A Question of Balance: The Relationship Between Daily Occupation and Sleep.’ In A. Green and A. Westcombe (eds) Sleep: Multiprofessional Perspectives. London: Jessica Kingsley Publishers.
Kroker, K. (2007) The Sleep of Others and the Transformations of Sleep Research. Toronto: Toronto University Press.
Macnish, R. (n.d.) The Philosophy of Sleep. Bibliolife. (Original work published 1834).
Paterson, L.M. (2012) ‘The Science of Sleep: What is it, What Makes it Happen and Why Do We Do it?’ In A. Green and A. Westcombe (eds) Sleep: Multiprofessional Perspectives. London: Jessica Kingsley Publishers.
Rechtschaffen, A. (1971) ‘The Control of Sleep.’ In W.A. Hunt (ed) Human Behavior and its Control. Cambridge, MA: Shenkman Publishing Company, Inc.
Scrivner, L. (2012) ‘That Sweet Secession.’ In A. Green and A. Westcombe (eds) Sleep: Multiprofessional Perspectives. London: Jessica Kingsley Publishers.
Sidis, B. (2010) An Experimental Study of Sleep (Kindle edition). Evergreen Review Inc. (Original work published 1909).
Zimmer, C. (2005) The Soul Made Flesh. London: Arrow Books.