Why Do We Sleep?

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Expert Verified By: Dr. Amita Fotedar, Ph.D


Just like eating, good night sleep is necessary for survival. There is nothing better than a good night’s rest. Sleep serves a significant role in our health and well-being. With so many myths around sleep, have you ever pondered why do we sleep? It is difficult to understand how critical sleep is to stay in pink of health and feel your best until you have not slept enough. But when you add the number of hours you spend sleeping, that amounts to about a third of your life so it must be good for something.

Sleep experts have proposed many theories about why we sleep. Here are what scientific evidences highlight so far about why we sleep. Plus how much sleep is needed on an average by us. The activity that takes place in our body and mind when we are asleep. What effects does insufficient sleep cause to our well-being, and how adequate and quality sleep is essential for optimal health? (Source).

Theories of Why We Sleep

Inactivity Theory

One of the earliest theories of sleep, sometimes called the adaptive or evolutionary theory, suggests that inactivity at night is an adaptation that served a survival function by keeping organisms out of harm’s way at times when they would be particularly vulnerable.

This inactive theory states that humans and animals that learnt to be quiet, calm and inactive in the times of vulnerability were more likely to survive and had an advantage over other organisms that were active. These animals did not have accidents (such as falling off a cliff because they did not see it in the dark) during activities in the dark, for instance, and were not killed by predators. These animals were thus favored by natural selection and this behavioral strategy presumably evolved to become what is now recognized as sleep (Source).

Energy Conservation Theory

This theory suggests that the major function of sleep is to alleviate an animal (including human) need for energy. For survival, humans and animals require energy pools. The more energy animals burn, the more fuel they need to take in. Hence, from an evolutionary point of view, more and more energy conservation is important as far as animals are concerned so that they do not have to make an attempt to constantly search for food. So, this fact supports the Energy Conservation Theory of sleep.

Scientific studies have shown that during sleep, the energy metabolism decreases by as much as ten percent. For instance, both caloric demand and body temperature alleviate during sleep, in comparison to when you are awake. Conservation of energy resources in animals and humans is supported by high-quality sleep (Source).

Restorative Theories

The Restorative Theory of Sleep, states that some processes during sleep restore damaged tissues and prepares the body for the next day. As per this theory, sleep enables the body to rejuvenate and re-energize itself.

Some supportive findings state that the major restorative activities and functions in the body like tissue repair, growth hormone release, muscle growth, and protein synthesis occur mostly, or in some cases only, during sleep.

This may involve clearing accumulated neurotransmitters (a group of chemical substances released by neurons that stimulate other neurons, muscle cell, or gland cell) from our brain as well as other tissue repairs that occur throughout our bodies. Together, deep sleep and REM (Rapid Eye Movement) sleep are often collectively termed as “restorative sleep.

Many other reinvigorating characteristics of sleep are related to cognitive abilities and brain.

For instance, while you are awake, neurons (also termed as nerve cells) in the brain release adenosine (a naturally occurring purine nucleoside that forms from the breakdown of adenosine triphosphate (ATP), a by-product of the cells’ activities.

The accumulation of adenosine (C10H13N5O4) in the central nervous system is one of the factors that trigger a perception in us that you are exhausted.

Sleep experts suggest that while you are asleep, this build-up of adenosine (C10H13N5O4) may encourage the “urge to sleep.” As long as you are alert and not asleep, adenosine builds-up. When you are in slumber, the body removes adenosine, and, as a consequence, there is an increased feeling of alertness when you are up and wide awake (Source)

Brain Plasticity Theory of Sleep

This theory states that brain shows growth and development while you are asleep. Sleep corresponds to alterations and changes in the structure and organization of the brain. You save or delete what you learned that day whether or not it’s important. This process termed as brain plasticity, and its connection to sleep has several critical implications. Research studies have reported that quality sleep plays a major role in brain growth and development in babies and young children. This throws light on the fact why these age groups have higher sleep requirements in comparison to adults with fully-developed brains. Infants sleep for about thirteen to fourteen hours per day, and about half of that time is spent in REM (Rapid Eye Movement) sleep – the stage in which most of the dreaming takes place. It will be wrong to say that adults will not be affected by this theory. Cognitive impairment seen in adults due to sleep loss highlights that quality sleep is important for healthy brains and well-being at every stage of life (Source).

Activity in the Brain and Body While Being Asleep

Why Do We Sleep? 1

Even though research by sleep experts continues to understand exactly why we sleep, one thing has been established without any further doubts: Good sleep affects the body and brain in distinctive ways that are not duplicated when we are awake (Source).

To have a better knowledge of the sleep effects on brain and body, it is beneficial to have an insight of sleep stages. There are 4 distinct stages of sleep: stage 1, 2, 3, and 4(REM Sleep). 

All these stages are helpful in restoring both the body and the brain which get exhausted during the periods we are awake. For instance, some of these stages will aid in learning and memory retention, while others are beneficial in revamping energy so you wake up feeling re-energized, rested and ready to take on the day.

Read more at Stages Of Sleep: How Sleep Cycles Works

Stage 1 of sleep is linked with both alpha and theta waves. It involves light sleep falling between being awake and being in a deeper slumber. It typically lasts for a few minutes and involves several physiological changes, including slower heartbeats, breathing, decrease in both overall muscle tension, eye movements, and brain waves.

It is also a non-REM stage and around fifty percent of the time spent asleep over the course of night is spent in this stage i.e., stage 2. This stage involves an advancement into a still-light-but-deeper state of sleep. Even though it is a light stage, the core body temperature drops, heart rate begins to slow, eye movements stop, muscles relax and the person asleep becomes unaware of the surroundings. This stage 2 repeats more than any other stage throughout a series of sleep cycles.

Stage 3 are extremely rejuvenating to the body and characterized by deep, restorative sleep. These are the stages where it is extremely difficult to wake up a person who is asleep. One feels highly energized and alert after waking up during these stages of sleep. Breathing and heartbeats become slow and brain waves slow down, and muscles remain relaxed. These stages occur for the longest periods in the first half of the night. These stages are sometimes referred to as SWS, or Slow Wave Sleep. Two groups of cells in the brain stem switch on to trigger SWS.

Stage 4 is characterized by REM (Rapid Eye Movement) sleep, which accounts for approximately twenty-five percent of a person’s sleep throughout the night. Somewhere between seventy and ninety minutes after falling asleep, you’ll enter REM sleep. After the first REM sleep cycle, you will experience more rounds of REM sleep at intervals of approximately ninety to a hundred and ten minutes. In this brain is bursting with activity, there is faster breathing, and increased blood pressure and heart rate. Most of the dreaming takes place during this stage. As the night progresses, deep sleep duration becomes shorter in length in comparison to the REM sleep that tends to become longer in duration. At the tail end of the entire night’s sleep, one’s sleep takes place in stages 1, 2, and Rapid Eye Movement (REM) sleep.

Negative Effects Of Sleep Loss

No matter the cause, sleep loss can cause a cascade of negative repercussions. Some unpleasant side effects that can occur when you are sleep deprived include;

  • It impairs the healthy function of the body’s main systems, including the cardiovascular, central nervous, digestive, endocrine, immune, musculoskeletal, and respiratory systems
  • It increases the risk of injury or fatality stemming from car, workplace, or at-home accidents
  • It increases the risk of infection and illness as well as the risk of serious health conditions such as cancer, diabetes, heart disease, stroke, and overall mortality
  • It interferes with personal relationships and may inhibit a healthy social life
  • It increases the risk of anxiety, depression, and other mental illnesses
  • It increases the risk of developing sleep disorders such as insomnia, narcolepsy, or sleep apnoea
  • It increases the likelihood of unintended weight loss or gain
  • It impairs workplace performance.

To sum up, inadequate sleep is associated with poor health and declined the quality of life overall (Source).

Read more at Sleep Deprivation: Symptoms, Effects, Treatments, & Prevention

The Bottom Line On Why We Sleep

“Why do we need sleep or why do we sleep” is a topic on which sleep specialists continue to conduct research and work to better understand exactly why do we sleep. Several theories have been proposed over the years, to explain the process of sleep—these include the Restorative Theory, the Inactivity Theory, the Brain Plasticity Theory, the Energy Conservation Theory, and the Reproductive Fitness Theory. To provide a succinct explanation, a combination of each of these theories gives an insight into why you sleep and why you need to sleep every day.

Sleep scientists say that regular and adequate amounts of quality sleep are vital for wellness and overall well-being – from physiological performance to cognitive function and emotional well-being. Sleep deprivation, on the other hand, is associated with a lot of negative effects like fatigue, clumsiness, daytime sleepiness, and increased appetite leading to weight gain. All these impacts can impair cognitive, emotional, physical, and social functioning.

Bottom line? Everybody needs sleep – it is as vital and critical as eating and breathing. While researchers continue with decades of study, it is still unknown why humans need sleep. However, to keep yourself alert, well-focused and energized during the day, make sure to get seven to eight hours of high-quality sleep every night.

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