Why do we sleep?Why do we do this thing that Shakespeare called The death of each day’s life, sore labour’s bath,Balm of hurt minds, great nature’s second course, Chief nourisher in life’s feast. (Macbeth)It may come as a surprise to you to know that, despite all the advances in sleep research and sleep medicine over the last couple of centuries, we do not have an answer for the question, why we sleep?We know much about what happens in that period of time that we call sleep, and we know much about what happens when we don’t get sufficient sleep, but neither of these tells us why we sleep. We have no explanation of why we need to spend between a fifth to a third of any 24-hour period asleep or why our sleep has the architecture it does. All complex organisms, basically it has a brain or something that resembles a brain, have a sleep-wake cycle. More recently, it has been claimed that primitive organisms‘ sleep’; Arnold C. Jellyfish caught snoozing give clues to origin of sleep. Nature News. 2017 Sep 21. however, this definition of sleep is based on the organism not moving for a particular period of time. Without having a brain, we can only say that behavioural inactivity is a surrogate indicator of sleep; this behaviour and does not necessarily equate to the sleep seen in more complex organisms. (Indeed, as an aside, the idea that not moving equates to sleep is the foundation of wrist-worn sleep trackers, which measure body movement. They use the fact that you move less when you are asleep to mean that you are actually asleep, but of course, we all know that you can sit or lie perfectly still whilst your brain is completely awake and functioning at a high level) Aristotle, who wrote the earliest surviving theory of sleep (On Sleep and Sleeplessness 350 B.C.) believed that sleep was the result of the heat from the brain being drawn towards the stomach to allow for digestion; this loss of vitality from the brain resulted in sleep. Aristotle theory of the link between sleep and digestion makes sense in terms of the evening sleeping, i.e., after having had your biggest meal. But this does not necessarily hold for your midday meal; the post-lunch dip does not need food to occur, although there are many examples in art of peasants asleep in the field at lunchtime (e.g. Pieter Bruegel the Elder, The Harvesters in The Metropolitan Museum)Like many ancient theories of science and medicine, this link between sleep and digestion persisted well into the 18th century (see for instance; To[bias] Venner, Via recta ad vitam longam: (London, 1628), Humphrey Brooke, Ugieine or A conservatory of health. (London, 1650), Francis de Valangin, A Treatise on Diet: or, The Management of Human Life (London, 1768))We have for centuries known that sleep/wake are affected by light levels, and it is true that humans are particularly ill-adapted to functioning in the dark (our nocturnal vision is particularly poor compared to most other mammals). It has been theorised that sleep evolved as avoidance behaviour, what Ray Meddis in his groundbreaking book the sleep instinct term the immobilisation theory of sleep, e.g., we can’t hunt at night and things that may attack us can so it is better to hide somewhere safe at night if we find somewhere safe then to conserve energy we might as well sleep. However, this theory is challenged by the fact that humans have had illumination, from fire, for approx. 400,000 years. Thius while this theory may have applied to proto-humans, it has not been relevant for the entire history of homo sapiens. In the 19th century, with the move away from the land into the city and the increased information and technological nature of life due to the Industrial Revolution, sleep began to be seen as something to be associated with the brain directly. “Those who think most, who do most brain work, require most sleep”. Memoirs of Richard Whately, Archbishop of Dublin With a Glance at his Cotemporaries & Times. In 2 Volumes. I By William John Fitzpatrick 1864)We now seem to have two different ideas of sleep 1) a time of recovery from physical labour, “sore labour’s bath”, 2) a time for ‘brain rest’. (Brain-rest; Being a Disquisition on the Curative Properties of Prolonged Sleep By James Leonard Corning 1885)However, for many of us, our modern lives require very little physical exertion, certainly compared to our pre-industrial forebears. Does this mean we no longer need sleep for physical repair and recuperation? We know that humans only physical grow during sleep, so is this the answer? Perhaps but it should be noted that whale and dolphin neonates have been observed not to sleep for the first few months after birth and yet grow at a prodigious growth rate during this period. (Lyamin O, Pryaslova J, Lance V, Siegel J. Continuous activity in cetaceans after birth. Nature. 2005 Jun;435(7046):1177-). This is the opposite of sleep in human children who sleep more during their early years when physically growing. This might be because humans would have to grow against gravity if they grew while awake while whales are supported in the sea. However, this difference does not explain why we humans need to sleep; we could just lie down for the first 13-18 years of our lives. More recently, we started to talk about sleep being essential for homeostasis, which is just a fancy word for ‘balance,’ i.e., keeping the brain and body working optimally. Sleep is important in keeping the endocrine, metabolic and immune systems functioning properly. But why given, the risks of predation and our vulnerability while asleep, has evolution not been able to find a way of modifying sleep so that it is shorter, or even abolish it altogether?In the 1980’s Prof. Jim Horne proposed the idea of core and optional sleep. (Why We Sleep: the Function of Sleep in Humans and Other Mammals”. (Oxford: Oxford University Press, 1988). Basically, this theory suggested that N3 sleep is the vital part of sleep and as this usually occurs in the first 3-4 hours of sleep, this period constituted core sleep in adults. The latter part of sleep, when we are predominantly in N2 or REM, he termed optional sleep, meaning essentially that staying asleep past core sleep was unnecessary. Although he has subsequently moved away from this theory, it is true that if you have one complete night without sleep, then your brain will try and make up all the missing N3 sleep. Animals sleep after they have satisfied all their biological needs, essentially if they have had enough food and water to survive, are in a safe place and, when appropriate, have taken the opportunity to ensure the survival of the species, then they will sleep. This is perfectly illustrated by the three-toed sloth, which was thought to need 16 hours of sleep a day; however, when someone bothered to study them in the wild, rather than observing them in captivity, they were found to only sleep less than 10 hours a day. The difference simply was that in captivity, they had all their needs met and so didn’t have to spend time looking for food, water, etc.Given the changes that have occurred; where we now have light and heat and food 24 hours a day, where we do little physical labour, where we are safe from predation and no longer vulnerable during the night, it is perhaps not a question of why we sleep but rather why we still need to sleep. Ray Meddis, in his controversial but influential book (The Sleep Instinct; Routledge & Kegan Paul, London; Boston 1977), suggest that in the modern world, a period of only a few thousand years, that maybe our sleep is designed to achieve something that is no longer necessary and is merely an instinct. So, does the need for sleep reside in the brain? It has been said that “sleep is of the brain and for the brain” We know that sleep plays an important role in the processing of memories and learning new tasks, and in the 70s and 80s, this led to the theory that sleep was mental downtime, computers, still very much in their infancy needed downtime and it was thought that our brains also needed this periodic downtime to cope with the influx of information. Given that our brains are vastly more sophisticated than even the most powerful modern supercomputer, this theory is, in hindsight, touching naïve. However, what is undeniable is that in humans, research has shown that sleep, in particular N3, Slow Wave Sleep (SWS), plays a crucial role in dealing with the events of the day, laying down memories and learning new tasks. This is important because what has undeniably changed over the last few years is the amount of information we are routinely exposed to, which we need to process during sleep. It does not matter that the vast majority of this ‘information’, funny cat videos and selfies, has no benefit to the development of the individual or the enrichment of society; our brain still needs to process it, even if it is just so that we can forget it. According to Eric Schmidt, CEO of Google, the amount of information is quite literally ‘mind-blowing’, speaking in 2010 “, Every two days now we create as much information as we did from the dawn of civilisation up until 2003”. It was estimated that in 2006 alone, the amount of ‘information’ created was 3 million times the amount of information contained in all the books ever written. This increasing information load that has to be processed during sleep is perhaps why we sleep and continue to sleep. .