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Why Do We Sleep?

Hayley Dawes
22 April 2022

Scientists don’t know why we sleep, but theories range from conserving energy to consolidating memories. Sleep is essential for good health. So, it’s no wonder we spend about one third of our lives sleeping. We have a biological need for sleep because it delivers numerous benefits, such as giving us more energy, reducing stress, and boosts the immune system.

There are four principal theories: the inactivity theory, the energy conservation theory, the restorative theory, and the brain plasticity theory. One thing is certain – not getting enough sleep can lead to disastrous health outcomes, from a weakend immune system to depression.

Many biological processes happen while we sleep:

  • The brain stores new information and gets rid of toxic waste.
  • Nerve cells communicate and reorganize, which supports healthy brain function.
  • The body repairs cells, restores energy, and releases molecules like hormones and proteins.

These processes are critical for our overall health. Without them, our bodies can’t function correctly. While we sleep, our bodies also repair cells and fine-tune hormone levels. “Sleep affects almost every tissue in our bodies,” explains sleep expert Dr Michael Twery.


The Inactivity Theory

Sometimes referred to as the “adaptive” or “evolutionary” theory, the inactivity theory was made back in the 1920s, before scientists had started sleep labs or discovered rapid eye movement. The idea is that for animals, night is a vulnerable time, when darkness makes it difficult to move safely and avoid predators. Lying still and quiet was a good way to escape danger until morning. Through evolution, the theory suggests that this strategy eventually morphed into what we now call sleep. The biggest issue – falling unconscious at night would have heightened vulnerability and made it nearly impossible to react to danger.


The Energy Conservation Theory

This theory suggests that the primary function of sleep is to cut down on the amount of energy an animal needs and how much it uses. Sleeping through the night (a time when hunting was challenging and hazardous anyway) was a good strategy to conserve energy. This theory is backed up by research that shows when humans are asleep, their metabolism slows by approximately 10%.


The Restorative Theory

After a long night of sleep, we often feel both rested and restored. Some scientists believe that physical and cognitive restoration is the reason why we actually sleep. Scientists have also discovered that most restorative functions – including muscle and tissue repair, protein synthesis, and the release of growth hormones – occur while humans sleep. But it’s not just your body that’s rejuvenated while you catch your Z’s. When you are awake, a chemical called adenosine accumulates in your brain. A result of cellular activity, it’s adenosine that is thought to make us feel tired. As more builds up during the day, the more tired we become. During sleep, the body clears adenosine from the brain, allowing us to feel refreshed and alert when the alarm goes off.


The Brain Plasticity Theory

One of the newest theories about why we sleep is based on research that suggests sleep is associated to changes in the structure and organisation of the brain. This theory, sometimes called the information consolidation theory, forwards the idea that during sleep, our brain sorts through the information we have acquired that day, getting rid of data we don’t need and storing the rest in long-term memories. Several studies support this idea, indicating that sleep deprivation has a negative impact on our ability to learn and recall information.

Whether sleep transforms our brains or conserves precious calories, scientists are unlikely to discover one definite reason behind sleep. The reason why we sleep is likely a combination of all of these theories.


Emotional Well-being

Sleep is necessary for emotional health. During sleep, brain activity increases in areas that regulate emotion, therefore supporting healthy brain function and emotional stability.

Research shows that sleep and mental health are intertwined. On the one hand, sleep disturbances can contribute to the onset and progression of mental health issues, but on the other hand, mental health issues can also contribute to sleep disturbances. 

Areas of the brain in which sleep increases activity include:

  • Amygdala
  • Striatum
  • Hippocampus
  • Insula
  • Medical prefrontal cortex

One example of how sleep can help regulate emotion occurs in the amygdala. This part of the brain, located in the temporal lobe, oversees the fear response. It’s what controls your reaction when you face a perceived threat, like a stressful situation. When you get enough sleep, the amygdala can respond in a more adaptive way. But if you’re sleep deprived, the amygdala is more likely to overreact.


Proper Insulin Function

Insulin is a hormone that helps your cells use glucose, or sugar, for energy. But in insulin resistance, your cells don’t respond properly to insulin. This can lead to high blood glucose levels. Sleep may protect against insulin resistance. It keeps your cells healthy so they can easily take up glucose. The brain also uses less glucose during sleep, which helps the body regulate overall blood glucose.



A healthy and strong immune system depends on sleep. Research shows that sleep deprivation can inhibit the immune response and make the body vulnerable to germs. When you sleep, your body makes cytokines, which are proteins that fight infection and inflammation. It also produces certain antibodies and immune cells. Together, these molecules prevent sickness by destroying harmful germs. This is why sleep is so important when you’re sick or stressed. During these times, the body needs even more immune cells and proteins.


Heart Health

Scientists believe that there is a link between heart diseases and poor sleep. The Centers for Disease Control and Prevention (CDC) says the average adult needs 7 hours of sleep a night. Getting less than that on a regular basis can lead to health problems, many of which can hurt your heart health.


What Happens If You Don’t Get Enough Sleep?

Without enough sleep, your body has a hard time functioning properly. Sleep deficiency is linked to chronic health problems affecting the heart, kidneys, blood, brain, and mental health.

Consequences of sleep deprivation can include:

  • Mood changes
  • Anxiety
  • Depression
  • Poor memory
  • Poor focus and concentration
  • Fatigue
  • Weakened immune system
  • High blood pressure


Sleep keeps us healthy and functioning well. It lets your body and brain repair, restore, and reenergise. If you don’t get enough sleep, you might experience side effects like poor memory and focus, weakened immunity, and mood changes.

Most adults need 7 to 9 hours of sleep each night. If you’re having trouble sleeping, talk to your doctor or a sleep specialist. They can determine the underlying cause and help improve the quality of your sleep.