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Why Do We Sleep?

Updated: Mar 20, 2021

Sleep is one of the most important functions of our day-to-day lives. Having a good night's sleep is essential for almost everything we do. Whether it's to have a productive day at work, or just hang out with friends, sleep can affect our moods, behaviours, and attitudes. That's why it is so important to pay attention to our quality of sleep every night. If you are consistently staying awake or waking up in the middle of the night, you might be at risk of having a sleep disorder.

What Happens When We Sleep?

Scientifically speaking, sleep is a periodic and natural loss of consciousness. When we are asleep, our biological clock, or circadian rhythm, determines when regular bodily cycles occur on a 24-hour basis. Some of these bodily cycles include wakefulness and body temperature. With regard to sleep, the brain remains active and has its own biological rhythm. These rhythms are known as the sleep stages.

There are two categories of sleep: NREM (non-rapid eye movement) sleep and REM (rapid eye movement) sleep. When we first start to doze off, our sleep begins in the first of four stages. The first stage is known as Stage 1 NREM. This stage is considered a transitional period from being awake to falling asleep. Usually, this stage is unremembered. In Stage 1 NREM, our muscles start to relax while our heart rate and brain waves begin to slow down. This is typical as we fall into a hypnagogic state of restfulness. This stage lasts for several minutes before our body shifts to the next stage.

During Stage 1 NREM, we may experience images that resemble hallucinations. These hallucinations are simply false sensory experiences, like seeing something that isn't there. These hallucinations are categorized as hypnagogic sensations. Hypnagogic sensations include incidents like seeing imaginary images on the walls or ceiling, or the common sensation of falling. These experiences in Stage 1 NREM mark the body's transition to light sleep.

The second stage, commonly identified as Stage 2 NREM, begins when our body moves into a deeper sleep. Our heart rate and brain waves continue to slow down while our muscles relax even more. This stage is differentiated from Stage 1 NREM by short bursts of high frequency electrical activity in the brain. These bursts of activity are scientifically known as sleep spindles. As we drift into sleep, our body temperature decreases and eye movements begin to diminish.

Image: Sleep Spindles Recorded on an EEG Machine.

The third stage, or Stage 3 NREM, is when our heartbeat, brain waves, and breathing come to their lowest points of the night. This is also when our muscles reach their ultimate point of relaxation. Stage 3 NREM is classified as a moderate deep sleep, also known as "slow-wave sleep" and our bodies become less responsive to external stimuli at this time. In this stage, we are hard to awaken, and this stage is the most important for physical recovery. This stage lasts approximately thirty minutes per cycle.

Finally, REM Stage is the fourth stage that we experience as we sleep. During this stage, our eyes move rapidly under our eyelids (hence the name "rapid-eye movement" stage), and we have our most vivid dreams at this time. An analysis of our brain activity shows that brain waves in REM sleep act almost as if you are awake. This explains why REM sleep is also known as "paradoxical sleep". Many areas of our brain are active during this time, yet our body appears relaxed, apart from some minor twitches. Our heart rate increases, breathing becomes irregular and rapid, and every half-minute or so, our closed eyes dart around energetically. These eye movements are indicators of the beginnings of a dream. Our dreams in this stage are often story-like and emotional, and we respond to the rich scenes as if we were watching it in real life.

What makes REM sleep even more unique is that our body ceases to move below the neck in this stage. This period of stillness is known as Atonia. So when we want to physically react to our dreams, our nerve impulses that send our body messages to move are blocked by a barrier, and we are left in a state of paralysis. Sometimes when we wake up from REM sleep, this immobility may linger. This effect is called sleep paralysis, but often does not last for more than a few minutes.

The sleep cycle repeats itself about every 90 minutes in young adults, and is shorter and more frequent as we grow older. The average adult goes through five cycles per night, and spends half of the night in Stage 2 NREM. This is because Stage 2 NREM is the most reoccurred stage in sleep.

Why Do We Sleep?

There are many reasons why sleep is necessary for our everyday lives, but here are our top five:

1. Sleep protects us. Many animals utilize sleep as a form of protection from their environment. For example, bears hibernating in the winter is a form of physical protection. Similarly, any species' sleep patterns tend to suit its ecological niche. Humans sleep at night in order to be protected from the cold and dark of the night. This is the most basic function of sleep.

2. Sleep allows us to recuperate. During our sleep cycles, our body's immune system is restored and brain tissues are repaired. Over the course of several hours, the neurons in our brain are allowed time to heal themselves and be ready to take on another day.

3. Sleep helps restore and rebuild memories. Throughout our day, we encounter many experiences, and sleep consolidates those memories by replaying them and strengthening those neural connections in the brain. While we sleep, our recent memories are shifted over from short-term memory to be stored in long-term memory. This is why it is recommended that you sleep after learning something new - it often enhances our memory!

4. Sleep fuels creative thinking. While we sleep, our dreams often inspire creative achievement. We may remember something in our dreams from the night before and be moved to pursue noteworthy accomplishments which we never would have thought of otherwise! Sleep overall boosts our thinking and learning skills.

5. Sleep supports growth. During Stage 3 NREM, our body releases hormones that are vital for muscle development. This explains why a good night's sleep is especially important for young children. The REM Stage and Stage 2 NREM also play an essential role in strengthening connections in our brain that build memories and help develop muscle memory. For example, driving a car becomes like second nature because your muscles and brain work together in deep sleep to build those muscle memories.

We've explained why you should pay attention to your sleep and why it's important to notice how you feel after waking up in the morning. Sleep is necessary for our daily lives and understanding the science of sleep's functions is the first way in which you can take control of your health and wellbeing.



1. Berger, F., Zieve, D., & Conway, B. Sleep and Your Health. MedLine Plus.

2. Zielinski, M. R., McKenna, J. T., & McCarley, R. W. (2016). Functions and Mechanisms of Sleep. AIMS neuroscience, 3(1), 67–104.

3. Rasch, B., & Born, J. (2013). About sleep's role in memory. Physiological reviews, 93(2), 681–766.

4. National Institutes of Health Brain Basics: Understanding Sleep. National Institute of Neurological Disorders and Stroke.

5. Eugene, A., & Masiak, J. (2018). The Neuroprotective Aspects of Sleep. MEDTube Science, 3(1), 35–40.

6. Denis D. (2018). Relationships between sleep paralysis and sleep quality: current insights. Nature and science of sleep, 10, 355–367.

7. Myers, D. G., & DeWall, C. N. (2021). Psychology. New York: Bedford, Freeman & Worth.

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