TL;DR: Sleep is vitally important yet the reason why we do it is still not understood. The inactivity, energy conservation, restorative and brain plasticity theories give us an idea of why we sleep.
Introduction
At the end of a long hard day, nothing feels better than to crash into bed and enjoy a nice long sleep. Almost everybody loves to sleep. It makes us feel better to do so. We feel like crap if we don’t get enough of it. Most people seem to genuinely enjoy it too.
Since sleep is so universal, as literally everybody sleeps, we don’t often ask ourselves why we do it. Why do we need to spend a quarter to a third of the day asleep? Why can’t we just stay up all the time? Why do we start going crazy and eventually die if we don’t sleep for long enough? This article will attempt to answer some of those questions.
So, Why Do We Sleep?
Believe it or not there isn’t a concrete answer to this question. Nobody really knows for sure why we have to sleep, other than for the fact that we don’t function very well when we don’t. Sleep is clearly very important, yet not something that we completely understand the reason for, at the time of writing this article. Below are a few scientific theories as to the reasons we sleep.
Inactivity Theory
The first, and arguably weakest, theory for why we sleep is the inactivity theory. Night time is dangerous. Our eyes can not see very well in the dark so we are more prone to either being attacked by nocturnal predators or injuring ourselves by tripping over unseen hazards. The inactivity theory proposes that it is advantageous to our survival to be inactive during increasingly vulnerable times, i.e. nighttime in the case of humans, so we evolved to sleep through the night. This theory has some merits to it, but doesn’t really explain why it seems biologically imperative for us to sleep. It also doesn’t explain why we need to be unconscious to sleep, as we would arguably be more able to respond to threats while conscious and can still be inactive while conscious, by just laying down without going to sleep.
Energy Conservation Theory
In addition to the night time being dangerous for us, it is also a difficult time for us to hunt. Our reduced ability to see makes it challenging for us to hunt other animals or forage for food. The energy conservation theory proposes that because the darkness of night time makes it difficult for us to find food, it makes sense for us to go into an energy conservation mode, i.e. sleep.
Research [1] has shown that energy metabolism is significantly reduced when sleeping, by as much as 10% in humans and even more in other animals, when compared to wakefulness. Both body temperature and caloric demand decrease while asleep. Combine this with the additional energy saving of physical inactivity and sleep can make sense for an energy conservation method. However, this still does not explain the severe adverse effects of not sleeping. If sleeping was just a method to conserve energy shouldn’t you just be able to eat more and then not have to sleep?
Restorative Theory
A more compelling reason as to why we sleep is to give our bodies time to restore and repair itself. Some functions of the body such as muscle repair, tissue growth, protein synthesis, and release of many of the important hormones for growth [2] have been found to occur primarily during sleep.
Sleep also gives the brain time to restore and clean itself. During wakefulness, the firing of neurons in the brian produces a byproduct called adenosine. Adenosine is an inhibitory neurotransmitter meaning it acts as a central nervous system depressant and inhibits many processes associated with wakefulness [3]. While you are awake adenosine continually builds up, inhibiting the processes of the brain more and more. Adenosine levels decrease when sleeping. The rising levels of adenosine in the brain during wakefulness, which inhibits brain function, and the subsequent removal of adenosine during sleep gives a compelling argument for adenosine removal being the reason why we sleep.
Additionally, sleeping has been shown to regulate many different hormonal functions within the brain. Leptin is the hormone that suppresses appetite and ghrelin is the counter hormone that increases appetite. Shorter nights of sleep have been associated with lower leptin levels and high ghrelin levels [3]. This leads to an increased appetite, which gives explanation as to why less sleep is often associated with higher BMI.
Clearly sleep plays a vital role in repairing the body, flushing out the build up of inhibiting chemicals from the brain and regulating important hormones. However, the big “why” still remains somewhat unanswered. There is currently no explanation as to why the body can’t do these things while awake.
Brain Plasticity Theory
The brain plasticity theory is a newer theory that proposes that sleep is necessary for neural reorganization and growth of the brain’s structure [2].
The highest level of brain development and learning occurs during the first few years of life. During a newborn’s average 16-18 hours of sleep a day, over 50% of the sleep has been found to be REM sleep [5]. This is quite high when compared to an adult, who is only in REM sleep for about 20% of the duration of a sleep session. The correlation between both a newborn’s large amount of REM sleep and sleep in general and their large amount of brain development seems to provide a good amount of support for sleep’s link to brain plasticity.
A study [5] performed on rats, who were deprived of REM sleep as babies found that they displayed higher rates of anxiety, disturbed sleep, reduced sexual activity, despair behaviour, reduced pleasure seeking and increased alcohol preference as adults, symptoms in line with those of Major Depressive Disorder. Brain measurements of these rats displayed a significant size reduction of the cerebral cortex and brainstem.
Sleep also has been found to have large effects on learning and memory. Learning and memory are often described in terms of three functions [6]
- Acquisition: Where new information is introduced into the brain
- Consolidation: Where the information becomes a stable memory
- Recall: The ability to access the information after it is stored, either consciously or unconsciously
Acquisition and recall occur when we are awake, however consolidation has been shown to take place during sleep through the strengthening of the neural connections that form our memories.
Studies [7] have found correlations between slow wave sleep (SWS) and declarative memory (e.g. remembering facts, like the capital city of a country) as well as correlations between REM sleep and nondeclarative memory (e.g. remembering how to do things, like riding a bike.)
The brain plasticity theory provides good evidence on the importance of sleep in the formation of our brains and our ability to learn and remember things, however, it still doesn’t actually answer the question as to why we sleep. It is still currently unknown as to why we need to go into an unconscious state in order for our brains to grow, develop and learn.
Conclusion
Sleep is complicated. We do not currently understand the reason why we do sleep, yet all the different theories provided effectively highlight the importance of sleep. Sleep has huge effects on the bodies ability to heal, grow and control hormone levels, and has huge effects on the brain’s ability to develop, learn and remain plastic. When looking to go UpRiver, getting enough quality sleep at night is crucial.
References:
[1] http://healthysleep.med.harvard.edu/healthy/matters/benefits-of-sleep/why-do-we-sleep
[2] https://www.ncbi.nlm.nih.gov/books/NBK482512/
[3] https://www.verywellhealth.com/adenosine-and-sleep-3015337
[4]https://www.sciencedirect.com/science/article/abs/pii/S0306452200002207
[5] http://mentalhealthsciences.com/publications/pdf/DangVu_PediatRehab_9-2-06.pdf
[6]http://healthysleep.med.harvard.edu/healthy/matters/benefits-of-sleep/learning-memory
[7]https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3768102/
