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why is sleep important

What Is Sleep and Why Is It Important?

We spend around a third of our lives sleeping. Many people would list it as one of their favorite ways to pass the time, while others struggle to achieve even a few hours of rest each night. But what exactly is this state slumber we enter each evening? And why is sleep so important? Here’s the low down on what happens at bedtime.

What is sleep?

Sleep is a naturally recurring period of daily rest. It is primarily activated by our body’s internal clock, which operates on different wake and sleep cycles throughout 24 hours. 

It is an altered state of consciousness that differs from being awake through our reduced capacity to respond to stimuli, plus our reduced sensory and muscle activity. 

When we rest, we are switched off from the world around us. Not entirely, because certain things externally (such as sounds or light) or internally (such as hormones or dreams) can cause us to wake up, but we are operating in Do Not Disturb mode. However, internally our body is still very active – but more on that below.

Why do we sleep?

Like so many of our everyday bodily functions, rest is something we often do without thinking about it. We know we need it, but do we know why?

The simple answer is that our body is designed that way. It needs this nightly recovery time to ensure specific processes occur, which only happens during slumber, so our body prompts us to rest when it requires us to.

These cycles of resting and activity are controlled through two different bodily processes:

  • Sleep/wake homeostasis: increased need to rest the longer you are awake. 
  • Circadian rhythm: differing periods of wakefulness and drowsiness throughout the day. 

If our sleep/wake homeostasis process were entirely in control, we would be at our peak of alertness and productivity in the morning and slowly become sleepier as the day progresses. However, our circadian rhythm is the process that varies the times we feel alert and tired, which differs from person to person. However, most adults tend to feel tired between 2am till 4am but also between 1pm till 3pm (hence why the Spanish traditionally have their daily siesta time around then).

Chemicals in your brain also work in different ways to keep your body awake or asleep. Research suggests that a chemical called adenosine slowly builds up in our blood throughout the day, causing us to feel drowsy once it reaches a certain level. Then this chemical dissipates while we rest, repeating the same process when we wake. One effective way to block adenosine receptors is caffeine, which is one of the reasons why we stop feeling tired when we drink coffee.

Sleep is not a waste of time. It supports general health and wellbeing. Sleeping has also been proven to enhance sport performance as it improves mood, boosts alertness, reduces injury risk, builds immunity, and repairs tissues. These are the reasons why everyone who likes to be physically active should prioritize it.

Kaisu Martinmäki, Senior Researcher, Polar Research Center

Why is sleep important?

Sleeping is vital for our everyday existence. Without regular rest, we simply can’t keep going. Here are some of the key reasons why sleep is so important.

Supports your general health and wellbeing

Research suggests that seven of the 15 leading causes of death in the U.S. are linked to ‘reduced sleep duration,’ which include cardiovascular disease, malignant neoplasm, cerebrovascular disease, accidents, diabetes, septicemia, and hypertension. Ensuring we regularly have a good night’s rest is critical to maintaining our overall health and wellbeing. 

Supports your immune system

Often when we feel unwell, we feel tired. Fighting an infectious disease means our body needs to increase the amount of rest to conserve energy. It does this by producing chemicals called cytokines, which fight the infection and induce slumber.

In our everyday life, a lack of sleep can also make us more susceptible to catching common viruses, such as a cold. It follows that a lack of rest can also affect how fast we recover.

Supports your nervous systems

Sleeping is vital for memory consolidation, information processing, and essential repairs to both our brain and body while we rest. Without this opportunity to exercise critical neuronal connections each day, our nervous systems can stop working effectively. 

Drowsiness, poor concentration, and memory recall can occur after a poor night’s recovery. After a few days of little rest, we can begin to experience hallucinations and mood swings, which result from our central and autonomic nervous systems not functioning correctly.

Supports your growth

Children and young adults require substantially more rest than people in their later stages of life because it is during deep sleep that growth hormones are released. In fact, throughout life, deep sleep increases our cell production and reduces the breakdown of proteins. So, these proteins can instead be used for repairing cells and damage from factors like stress and UV rays. Hence why, it’s called ‘beauty sleep.” 

Supports your mental health

There is a strong correlation between the amount of rest we have and our mental health. Sleep deprivation has the potential to trigger anxiety, paranoia, and hallucinations. On the other hand, regular good quality sleep can significantly enhance our mental wellbeing, boosting our capacity for humor, happiness, and self-confidence.

Tracking sleep with Polar Sleep Plus Stages™ makes the amount, quality, and timing of sleep visible. I hope it helps people to understand how they sleep and how their lifestyle and habits affect their sleep.

Kaisu Martinmäki, Senior Researcher, Polar Research Center

What happens when we sleep?

Until the 1950s, we generally thought of slumber as a passive activity. That our body and mind indeed did switch off completely, entering a dormant state while we snoozed away. These days, we now know that though our muscle and sensory activity may be reduced, our brain is very active. Here’s a look at what happens across our whole body while resting.


The one thing we all know we do while we rest is dream. Although dreaming can occur in any sleep stage, it occurs mainly (and most intensely) during REM sleep, where we often experience our most bizarre or immersive dreams.


Our breathing changes while we rest, depending on what stage of sleep we are experiencing. During non-REM sleep, our breathing slows, becoming especially slow during Stage 3 (deep sleep). However, during REM sleep, our breathing rate can pick up and become quite irregular.

Heart rate

Like breathing, our heart rate also slows during non-REM sleep but quickens during REM sleep to almost the same as when we are awake.


As we drift off into slumber, our muscles gradually become increasingly relaxed during the non-REM stages of sleep. Then during REM sleep, the muscles in your body become paralyzed to ensure your legs and arms don’t move around in response to your dreams. However, your eye muscles stay active, which is why this sleep stage is called REM (for rapid eye movement).

Brain activity

The activity in our brain while we rest enhances the functioning of our brain when we are awake (hence another reason why sleep is so important). Vital cognitive functions, such as our memory consolidation, happen when we are resting, meaning that we wake up ready for a new day with an updated brain that’s optimized and ready to go.

During non-REM sleep, our brain slows down along with our heart rate and breath. However, during Stages 2 and 3, we can experience bursts of brain activity. During REM sleep, our brain is very active as we begin to dream.

So, there we go. Some pretty convincing reasons why sleep is important and should be a priority in our everyday lives. If you’re now wondering how to create the perfect environment for rest so you can maximize your snooze time, try these expert tips

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Please note that the information provided in the Polar Blog articles cannot replace individual advice from health professionals. Please consult your physician before starting a new fitness program.

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