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Why Do We Need To Sleep?

8th February 2019

Slumber Centre

Sleep is one of the natural habits that all living things share, humans and animals alike. It’s a time when our brain mostly shuts down, leaving us vulnerable to predators and unable to pursue any of the other activities crucial to our survival, so whatever the benefits are must be worth the risks. We know that sleep is crucial for our wellbeing; it’s been proved that prolonged lack of sleep can result in hallucinations, physical health problems and, in extreme cases, psychotic episodes. But exactly what happens during sleep, and why it’s so essential, remains largely a mystery.

What do we know about sleep?

From sleep research and experiments conducted on animals, researchers have deduced that the primary function of sleep is probably related to the brain.

Coordinating brain activity

There are some theories that sleep is just something that happens when neurons work together in the brain; it’s been noted that neuron networks grown in laboratories show noticeable waves of activity that are similar to patterns of waking and sleeping. Marcos Frank, a neuroscientist at the University of Washington, reasons that this could be a solution of nature to ensure all parts of the brain are active and resting at the same time.

Sleep pressure

Biologists generally refer to our need for sleep as ‘sleep pressure’; the longer we’re awake, the more ‘sleep pressure’ we build up and the stronger our urge to rest becomes. Studies suggest the ‘sleep pressure’ is caused by a chemical called adenosine which accumulates in the brain while we’re awake and then reduces during sleep. Caffeine acts an adenosine blocker so it can’t build up while we’re awake, which is one of the reasons caffeine makes us feel more alert.

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The stages of sleep

There are two main sleep stages: REM (rapid eye movement) and non-REM, although the non-REM stage can also be broken down into a further 4 stages. You’ll move through all these stages in 1.5 to 2 hours, and then a new cycle begins. Here’s what your body gets up to in each of the stages...

Stage 1

In stage 1 you sleep very lightly and can easily be woken; however, if you are woken you may feel like you were never even asleep to begin with. If uninterrupted, Stage 1 will last for five to 10 minutes when you first fall asleep. During this stage, the eyes move slowly, and you may feel a sense of falling followed by muscle contractions.

Stage 2

In stage 2, eye movement stops and brain waves slow down. Heart rate slows and body temperature drops in preparation for sleep.

Stages 3 & 4

As the body moves into deep sleep, the brain produces slower brain waves called delta waves. It’s harder to rouse someone in this stage, and if they are woken they can feel disorientated for several minutes. It’s during these deep sleep stages that most healing is done; the body repairs and regrows tissue, builds bone and muscle, and strengthens the immune system.

Stage 5 (REM sleep)

REM sleep is where most dreaming occurs. Brain activity increases, heart rate speeds up and eyes remain closed but move rapidly from side-to-side. REM sleep is typically reached around 90 minutes after falling asleep, with the first REM stage lasting around 10 minutes and each following stage getting longer until the final one lasting around an hour.

The exact purpose of REM sleep is unclear, but we do know that it seems to be less essential than deep sleep; the effects of REM sleep deprivation are less severe. The percentage of REM sleep we have is highest during infancy, declining in adolescence and then again around the age of 60.

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What happens during sleep?

Just because we’re resting, doesn’t mean our body is! A lot of important activity goes on while we’re asleep, which is why it’s so crucial we get enough.

Memory processing

During sleep our memory processes all the information it’s collected during the day. This is particularly important for long-term memory creation, as the brain selects important information from your day’s memories and files them away for future use.

Growth and repair

During sleep the body releases growth hormone, which helps your body to grow and repair itself. This is one of the reasons children require more sleep, to ensure they’re releasing enough growth hormones during this key stage of development.

Stress level management

During the first few hours of sleep our cortisol, or stress hormone, levels decrease to make sure we stay at a healthy level of relaxation. We need cortisol to stay alert and give us energy during the day, but too much of it can lead to health issues, which is why we need sufficient sleep time to allow levels to drop.

Take a break from the toilet

We wouldn’t get much quality sleep if we had to go to the toilet as much at night as we do during the day. At night an anti-diuretic hormone called ADH is released to suppress our need to pee - pretty smart, right?

Fighting infection

During sleep our immune system releases proteins which are key to fighting disease and inflammation. This is why you sleep more when you’re ill, so your body can produce the proteins needed to fix itself.

Muscle paralysis

During REM sleep the body paralyses its voluntary muscles to prevent the sleeper from acting out their dreams and potentially injuring themselves. If a sleeper is woken up during REM sleep they may stay paralysed for several minutes; this is what is known as sleep paralysis.

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How much sleep do we need?

Everyone needs a healthy amount of sleep, but the actual number of hours we need varies throughout our life.


We need most sleep during infancy, when the most development and growth takes place. Newborns up to a year old can sleep for around 15 hours a day, with this number slowly decreasing through childhood. By 12 years old children should still be getting around 10 hours sleep a night, with teenagers needing 8-9 hours.


Despite the widely recommended average of 8 hours of sleep a night, in adulthood there is an acceptable range of 7-9 hours, which means you might be doing better than you think! This remains the same until around 65, when the range reduces again to 7-8 hours per night.

There are differences between the age groups in the way we sleep, too. Children easily drift into deep sleep and spend more time in this stage, when most growth hormone is released. As you age, you’re likely to spend more time in light sleep stages.

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Effects of sleep deprivation

We might not know exactly why we need to sleep, but we do know there are lots of bad effects of sleep deprivation. Even after one poor night’s sleep, we usually feel unfocused, run down and short-tempered, and these effects only get worse over time.

Reduced focus

After several sleepless nights, you’ll find it increasingly harder to concentrate, start to feel down or more stressed as your cortisol levels rise, and you’re more likely to forget things. Your immune system will be weakened, and you’ll also risk causing injury to yourself through clumsy accidents, particularly if you drive.

Skin problems

There’s also some truth in the phrase ‘beauty sleep’; not getting enough sleep can almost immediately leave you with bags under your eyes and uneven skin tone. Cortisol, the stress hormone that drops during sleep, also breaks down collagen, the protein that keeps your skin smooth, resulting in loose skin and premature wrinkling.

Physical impact

If you continue to lose sleep over time, more serious long-term health problems can emerge. High blood pressure, heart attacks and diabetes have all been linked to chronic sleep deprivation, and those who don’t get enough sleep are more likely to gain weight over time.

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While there’s still lots to discover about what happens during sleep and why we need it so much, what is undeniable is that regular high-quality sleep is essential to our physical and psychological wellbeing. Find out more about the signs of sleep deprivation and how to solve them on our blog, and consult a medical professional if you’re concerned about your sleep.