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Why Do We Sleep?


There hasn’t been a single answer to this question yet because sleep is incredibly complex with numerous functions and processes occurring during sleep. Although physically quite inactive, our mental activity is not to be ignored. Our minds prepare us for future learning, we store information from short-term into long-term memory, we dream – here’s where we get emotionally involved with the information our brain is dealing with.

Our energy levels decrease as the metabolism slows down, nerve cells get ‘cleaned’ from harmful substances, neurotransmitters and hormones are replenished, the immune system is strengthened, cells and tissues are restored and rejuvenated.

In the morning, we feel rested, emotionally stable, energetic, and overall good. This is how it should be in healthy people. Various sleep disorders and disturbances helped scientists see what happens when we get insufficient sleep or insufficient quality sleep. They have conducted research on what happens if a person is deprived of only one sleep stage. Consequences of poor or no sleep clearly show us why we need to sleep in the first place.

Here we give an insight into current sleep theories and popular findings in the process of searching for an answer to ‘Why do we sleep?’

Theories of why we sleep

Although nobody can agree on one single theory as to why we sleep, it is good to have an insight into current theories as they are products of lots of sleep studies and research. Combination of two or more theories might give us a more accurate answer.

Inactivity theory (Evolutionary theory, Adaptive theory)

This theory suggests that sleep was developed in the process of evolution. Scientists argue that animals who were inactive at night were not as vulnerable to predators as those that were moving around. Moreover, nighttime and low visibility increase the dangers of falling down or hurting themselves. It is thought that sleep was just an evolutionary appropriate behavior which proved the most effective and which thus developed further and remained a necessary behavior in most animals.

This theory seems unsupported by strong arguments – for example, how come an unconscious animal is better protected than a fully alert animal? If safety was the main reason we sleep, why would we be so unaware of our surroundings and possible approaching dangers during sleep?

Energy conservation theory

This theory may be partly related to inactivity theory. It states that considering low energy use during sleep, the main reason for sleep might be energy conservation. Competition for the source of energy could be linked to the survival of the fittest. Also, sleep helps living beings conserve energy at times when they aren’t able to find food easily (at night or in winter).

Our brains consume about 20% of our energy which is significant, and reducing brain activity at least a bit may be beneficial for overall energy saving. During sleep, energy expenditure goes down by 5-10% which is enough to preserve some of the valuable energy. One of the strongest points of this theory is the fact that cold-blooded animals sleep a lot less than warm-blooded animals.

However, even though energy is partly conserved during sleep and metabolism is slowed down, there are numerous other physical and mental health benefits to sleep which can’t simply go ignored in favor of energy preservation theory as the sole reason for sleeping.

Restorative theory

Growth hormone is released during sleep, and it helps repair and restore cells and tissues in our body. In children, this hormone promotes the growth of the body, which may be an answer to why children need so much sleep.

Studies have shown that complete deprivation of sleep for a certain period can result in death of animals, as they completely lose the defense mechanisms of their immune systems. Our brains need maintenance during sleep because the activity of our nerve cells results in by-products which accumulate more and more over our waking time. The name of this by-product is adenosine and its accumulation is one of the things that make us sleepy. We need to stop producing adenosine in order to clean it up from the brain, and sleep is the way to get this done.

It should be noted that caffeine ‘masks’ our tiredness by blocking the effect of adenosine. This can be bad for our brain health as we lose touch with our natural sleep needs and fail to let our brain do the maintenance, allowing way too much adenosine to build up.

Sleep also helps our emotional stability by letting amygdala, the fear and emotion center in the brain, get refreshed. With sleep loss, our muscles and tissues aren’t restored and memory isn’t consolidated, which makes it more difficult to concentrate and perform physical activity.

Brain plasticity theory

This theory relates to changes in how our brain is structured and organized. We know that in children most brain development occurs during sleep; however, sleeping is beneficial for brain structuring of the adults, too. Some synapses are strengthened and some are not – depending on the importance of information our brain is processing.

As deep, slow-wave sleep is responsible for one part of memory consolidation, rapid eye movement (REM) sleep additionally employs emotional response to learning. One study conducted in Ottawa has shown that French-learning participants had more REM activity and dreamed about communicating in French. Those participants who didn’t experience increased REM made the least progress during the course. Knowing what things are and what they are called is known as declarative memory.

REM also seems to be involved in visual learning, while light sleep stages (stage 1 and stage 2) are important for motor learning – learning how to do things – this is called procedural memory. Brain processes information which requires motor skills like successfully coordinating and performing tasks (swimming, riding a bicycle).

What happens if you don’t have enough sleep?

Sleep scientists have linked lack of sleep to a shorter lifespan and an increased risk of developing a type of dementia, like Alzheimer’s disease, later in life. Chronic sleep deprivation is linked to diabetes, cardiovascular diseases, motor disorders, and weak immune system. Bad learning and memory skills, frequent migraines and a higher risk of anxiety are what strikes those who don’t have enough REM sleep.

Even one night of poor sleep leads to impaired cognitive abilities the following day. So if you cut down on your sleep in order to work more, you will end up making more mistakes and working less because you won’t be as fast as you could be. Sleeping is a part of our natural circadian rhythm and it should be respected as such.

Minding your sleep habits and taking care of the time you go to bed and wake up, making sure you have enough hours of sleep is crucial to healthy sleep. There are many things you can do to improve the quality and quantity of your night’s sleep.

Good night’s sleep will ensure you are healthy and reduce the risk of errors, including those during drowsy driving, which make one of the main causes of traffic accidents.

Additional resources

  1. Why Do We Sleep, Anyway? Healthy Sleep. Harvard. http://healthysleep.med.harvard.edu/healthy/matters/benefits-of-sleep/why-do-we-sleep Accessed December 23, 2018.
  2. De Koninck J, Christ G. Language learning efficiency, dreams and REM sleep. Psychiatric Journal of the University of Ottawa. https://www.ncbi.nlm.nih.gov/pubmed/2374794 Accessed on December 23, 2018.
  3. Sleep, Learning, and Memory. Healthy Sleep. Harvard. http://healthysleep.med.harvard.edu/healthy/matters/benefits-of-sleep/learning-memory Accessed December 23, 2018.

The information on this website is not intended to replace a one-on-one relationship with a qualified health care professional and is not intended as medical advice. Read our full medical disclaimer.

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