For a long time, sleep has been a subject of speculation and thought, and it’s only recently that scientists came up with objective and systematic ways of studying sleep. The most popular technology in this area is electroencephalograph (EEG) which has enabled scientists to study and quantify electrical activities and patterns produced by the brain of a sleeping person. Despite these discoveries and decades of research, one question still perplexes scientists and researchers when it comes to the subject of sleep - why do we need to sleep?
Mander Bryce, a neurologist at Neuroimaging Lab and UC Berkeley’s Sleep says, “There have been a lot of medical researchers who have put forward theories about why we sleep, but most of these theories are largely incomplete. Sleep affects almost all organs in the body and as such, it’s hard to study. There are numerous theories of sleep, but none of them has been proven correct.”
However, there’s no clear cut answer to this challenging question, one thing these experts all agree on is that sleep plays a very critical role in our overall health and wellness. Also, there are some rather obvious clues that could help explain our need to nap on a daily basis. One such clue is the fact that, for most of us (if not all), a goodnight’s slumber leaves us feeling much better in the morning, whereas a lack of a decent night’s rest leaves one feeling much worse. Several theories have also been put forward, which attempt to further explain why sleep is such a great necessity in the human life, and below are some of the more promising ones:
Repair and Restoration Theory of Sleep
Based on a long held belief that somehow sleep “restores” something that is spent in our bodies while we are awake, the repair and restoration theory is perhaps one of the most popular theories of sleep. According to it, sleep affords the body the much needed opportunity it requires to repair and rejuvenate itself, which involves restoring and revitalizing the physiological processes which keep your body and mind healthy and functioning properly.
Lately, this theory has gained ample support following considerable empirical evidence gathered from human and animal studies. Several of these experiments suggest that in some ways, sleep allows the body’s immune system to function better. For instance, in one experiment it was shown that people who regularly had less than 7 hours of sleep a night were approximately three times more likely to fall ill when exposed to the common cold virus than those who had 8 or more hours of sleep every night. The theory is further supported by research findings which have shown that many of the major body restorative functions mostly or, in some cases, only occur during sleep. These include restorative functions such as, protein synthesis, tissue repair, muscle growth, and growth hormone release.
Other than the body, another segment of the repair and restoration theory focuses on the rejuvenating aspects of sleep that is specific to the brain. The theory explains that sleep has an essential function in relation to production of adenosine; a chemical produced by neurons in the brain as a by-product of regular cellular activity. When one is awake, adenosine accumulates in the brain and scientists think that this build-up may be one of the factors that contributes to our perception of feeling tired and eventually promotes the "drive to sleep. However, during sleep, the body gets a chance to clear this chemical from the system, and as a result, the feeling of tiredness goes away which in turn leaves you feeling more alert when you wake up.
Evolutionary Theory of Sleep
Also referred to as the Adaptive Theory or Inactivity Theory, the Evolutionary theory is one of the earliest theories that tries to explain the necessity of sleep. It suggests that periods of inactivity at night or during the daytime is an adaptation which served a survival function by ensuring that animals remained out of danger during times of vulnerability. That is, according to the theory, animals that were able to stay quiet and still during those times when they were particularly vulnerable had a greater chance of survival than those species which remained active. Sleeping animals were able to evade predators, stay safe from possible accidents that could happen during activities in the dark, and other risks that affect mobile creatures. As a result, through natural selection, these animals survived in greater proportions and eventually this behaviour or survival tactic presumably evolved to become what is now recognized as sleep.
However, the evolutionary theory of sleep is not widely accepted and several flaws in this hypothesis have been cited by those who challenge it. In a simple counter-argument, some people argue that when an animal is sleeping, it is not able to respond promptly to potential threats, and this makes it extremely vulnerable and the perfect prey for predators. Therefore, it would be much safer for an animal to remain conscious, as this would enable it to react quickly to emergencies (even if it’s just lying still in the dark of the night). In fact, some researchers have used this counter-argument to point out that, the vulnerability of a sleeping animal is reason enough to believe that sleep must be serving some other essential function. Otherwise, this careless behavior would have been phased out by evolution centuries ago.
Brain Plasticity Theory
Brain Plasticity Theory is probably one of the more recent, and rather compelling, theories of sleep. It is based on research findings, which suggest that sleep is correlated to changes in the organisation and structure of the brain. In earlier days, the brain was perceived as a static organ, but many studies have since disproved this notion by showing that, the brain has the ability to adapt and change over time. The term “brain plasticity” was coined to refer to this extraordinary ability of the brain to change throughout an individual’s life; and proponents of this theory believe that sleep contributes importantly to the processes of brain plasticity.
To start with, the brain plasticity theory argues that, people sleep so as to process the information they have acquired during the day and consolidate new memories. It suggests that, when one is asleep, the brain does not rest idly, but rather, it utilises this time to sort out and review the activities and information absorbed throughout the day, and cements these things into long-term memory. Support for this hypothesis has been drawn from several studies, which demonstrate that lack of sleep has a negative impact on the ability to remember information. For instance, in one experiment a number of volunteers were given aptitude tests such as remembering a sequence of patterns presented to them on a computer. Half of the volunteers were shown these patterns during the morning session, and the other half in the evening, after which, their memories were tested by the team of researchers. For the morning volunteers, the test was done after a full day of being awake while the evening learners were tested after a night's sleep; and sure enough, the individuals allowed to sleep had better recollection of the test patterns.
The argument presented above can also be used to explain why sleep is vital in the brain development of infants and young children. That is, the same reasoning can be used to argue that, the reason why infants spend a huge part of their lives sleeping (about 13 to 14 hours of sleep every day) is because at this stage, babies are constantly learning about the world around them and hence, they require more deep sleep for them to process all this information.
Energy Conservation Theory
In natural selection, competition for and effective utilisation of energy is extremely important – in that, if a species is able to make use of its given energy resources in a slightly more effective manner, it would have a great advantage over similar species. Using this line of thought, the energy conservation theory tries to explain why we need to sleep by suggesting that sleep has something to do with saving an individual’s energy. In general, it proposes that the primary function of sleep is to lower ones demand for energy as well as reducing the amount expended during part of the night or day; especially during those periods when it’s least efficient to look for food. This would make some sort of sense, if you thought about the evolutionary ancestors’ struggle to find enough to eat, and them using sleep as a way of cutting down their energy consumption, so as to be able to make the most of their limited food.
Research has shown that, when humans are asleep, their energy metabolism is significantly lowered by about 10% (this figure is actually higher in other species). Generally, our body temperature and caloric demand tends to drop during periods of sleep, and increases when we are awake. It is such evidence which gives support to this intriguing hypothesis that, indeed, sleep does play a role in helping organisms to conserve their energy resources. Some researchers actually consider this idea to be part of and, correlated to, the evolutionary theory of sleep. However, there are those who doubt that this kind of reduction (the 10% drop in energy metabolism) can make a significant change in survival or even the overall energy consumption rates.
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