Everybody will agree that sleep is an important aspect of our life, important for our physical and mental health and also a great pleasure! It is so important that during our life we spend almost a third of our time sleeping. However, we still don’t really know why we do it… despite years of research about it, the true function of sleep is still uncertain. One of the main theories on sleep states that the brain needs it. The question then is how is sleeping useful to our brain? It is now proved that sleep helps in memory consolidation and, therefore, learning. A study published some years ago by Giulio Tononi (Wisconsin University) showed that during sleep the brain eliminates redundant and useless connections. Furthermore, a recent experiment by Robert Stickgold (Harvard University) showed that if students have the possibility of sleeping between two tests, they will perform better on the second one. While sleeping, the brain appears to repeat a pattern of neuronal activation that occurred when the person was last awake, as if it is trying to reinforce the traces of the information recently learnt. According to these findings, the purpose of sleep would be helping us to remember what is important whilst letting us forget what’s not. Sleep has physiological effects too; indeed, prolonged sleep deprivation can lead to death, as proven in experiments conducted by Rechtschaffen (University of Chicago). In these experiments, rats were sleep deprived by placing them on a disk suspended over a tank of water. If the rats fell asleep, they would fall in the water and wake up again. After two weeks, every rat was dead. However, necropsies on the animals didn’t find anything significantly wrong with them. All the organs and vital markers were not altered, and the only reason of death was exhaustion, that is lack of sleep.
PHYSIOLOGY OF SLEEP
The sleep-wake cycle is controlled by two internal influences: sleep homeostasis and circadian rhythms. Sleep, like other body conditions such as blood pressure and temperature, is under homeostatic control; in other words, the body maintains these parameters in a steady state. From the time we wake up, the homeostatic drive for sleep accumulates until late evening, when we will eventually fall asleep again. One neurotransmitter, adenosine, seems to be the sleep-inducing chemical. In fact, the level of adenosine rises consistently when someone is awake, resulting in an increasing need for sleep. Conversely, the level of adenosine decreases during the night, satisfying the need for sleep. Some drugs (like caffeine!) act on the adenosine receptor, disrupting this process to some extent. Circadian rhythms are cyclical changes occurring in a 24-hour period driven by the brain’s “biological clock”. This consists in a group of neurons in the hypothalamus, called the suprachiasmatic nucleus (SCN). The physiology and behaviour regulated in these cycles are synchronised to the external physical environment and social schedules. The strongest synchronising factors are light and darkness, the external stimuli that set the “biological clock” and determine when we need to fall asleep and wake up. Although we think of sleep as a period where we shut down, sleep is actually an active physiological process. There are two types of sleep: rapid eye movements (REM) and non REM (NREM) sleep, characterised by distinct brain activities. NREM sleep, characterised by a reduction in physiological activity, consists of 4 stages:
- Stage 1: the transition from being awake to falling asleep – this is characterised by slow brain waves and diminished muscular activity.
- Stage 2: period of light sleep where eye movements stop, brain waves become slower and spontaneous periods of muscle tone are mixed with periods of muscle relaxation.
- Stages 3-4: these are characterised by slow brain waves known as delta-waves interspersed with small faster waves. Sleep is deep, with no eye movements and decreased muscle activity, although movement is possible.
REM sleep is a paradoxical change in brain activity: the brain is extremely active, its waves are fast and desynchronised, similar to those characteristic of the waking state. Breath becomes irregular, eyes moves rapidly and limb muscles become temporarily paralysed. This is the stage where most dreams occur. The role of each phase in overall health is still uncertain; however, striking a good balance between the phases appears to be crucial in achieving beneficial sleep. A complete sleep cycle lasts about 90-110 minutes, and is repeated from 4 to 6 times every night. The composition of each cycle is not constant during the night (REM sleep increases after each cycle), and also changes during the life of an individual, with children having significantly longer periods of REM sleep compared to adults.
Sleep deprivation, chronic or acute, is the condition of not having enough sleep. The short-term consequences of this condition are well-documented, including diminished cognitive performance, impaired memory and low levels of alertness. Prolonged periods of inadequate sleep have cumulative side effects. In one of the most extensive studies on human sleep deprivation, subjects were restricted to 6 hours of sleep per night for two weeks. Subsequently, in cognitive and motor tasks, they performed as poorly as subjects who were entirely deprived of sleep for two consecutive nights. In the long term, poor sleep habits negatively impact the functions of several organs (such as heart, lung and kidneys), on metabolism and weight control, immune responses, sensitivity to pain, as well as mood and cognitive functions. Sleep deprivation is also a risk factor for depression and substance abuse, and it has been linked to increased risk of diabetes, heart disease, obesity, and certain forms of cancer. Obviously, the consequences on humans of total sleep deprivations are not well documented, and our knowledge about this comes from only a few studies, world-record attempts and distressing stories like the one told by the psychotherapist John Schlapobersky, who was tormented with sleep deprivation: “I was kept without sleep for a week in all. I can remember the details of the experience, although it took place 35 years ago. After two nights without sleep, the hallucinations start, and after three nights, people are having dreams while fairly awake, which is a form of psychosis. By the week’s end, people lose their orientation in place and time — the people you’re speaking to become people from your past; a window might become a view of the sea seen in your younger days. To deprive someone of sleep is to tamper with their equilibrium and their sanity”. People who are left without sleep for long periods of time usually recover after a few days. So far, no human death has been attributed to forced or intentional wakefulness. However, some rare instances where humans are literally unable to sleep have ended in death. This is known as fatal familial insomnia (FFI), which is an extremely rare prion brain disease, which results in total inability to sleep, dementia and ultimately death, within a time frame of 7-36 months. The disease progresses from insomnia, hallucinations, temperature fluctuations, to complete loss of sleep, weight loss, dementia, irresponsiveness, and eventually to sudden death. These symptoms suggest that prolonged periods without sleep would end in death by disrupting critical functions such as those related to metabolism. The person becomes hypometabolic, and cannot appropriately manage energy intake and expenditure, so that the energy is wasted.
Sleep is a fundamental physiological function that serves vital roles to the organisms. All animals sleep, from birds to fish, and some of them can do it with one hemisphere at a time to maintain a certain level of alertness. Many sleep disorders can lead to mild sleep deprivation that will have negative repercussions on cognition and physical health. In the most severe cases, sleep deprivation can eventually cause dementia and death. It is therefore important that we take care of our sleep, keeping in mind that is not only the quantity that matters, but most importantly the quality of the sleep and in what sleep phase we are in when we wake up. That is the case of when we are forced to wake up and we end up feeling sleepy all day, opposed to when we naturally wake up, maybe at the same time of day, but feel great and full of energy. We can roughly calculate how much time we should sleep if we want to wake up in a better state by considering the length of the sleep phases. However, no matter what, I find myself agreeing with Wilson Mizner, who wisely said: “The right amount of sleep required by the average person is 5 minutes more”. Do you want to find out if you suffer from a sleep disorder? Take this simple test: http://www.edinburghsleepcentre.com/sleep_disorders/online_questionnaire.htm
http://ngm.nationalgeographic.com/2010/05/sleep/max-text http://sleepfoundation.org/sites/default/files/SleepWakeCycle.pdf https://en.wikipedia.org/wiki/Sleep_deprivation http://io9.com/can-you-die-from-sleep-deprivation-1684235719