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Why do we sleep?

A lot is still unknown about the purpose of sleep.1 However, we know that quality sleep is as essential to survival as food and water.2 Sleep is so important that we spend a third of our lives sleeping2 – so why do we sleep? Here, we’re exploring the important functions sleep plays in protecting and maintaining our physical, psychological and behavioural wellbeing.

What is sleep?

Sleep is a naturally recurring state of mind and body, characterised by altered consciousness.3 We differentiate sleep from wakefulness by a state of muscle relaxation, reduced perception and diminished reaction to environmental stimuli.3 When we sleep our body experiences two different physiological ‘sleep states’ – Rapid Eye Movement (REM) and Non-REM (NREM).4

Sleep stages

A typical night involves 4 to 6 repeated cycles of NREM and REM, each lasting approximately 90 to 110 minutes. First comes Non-REM which is where we spend about 70% of our total sleep.5 Non-REM can be separated into 3 stages and each stage results in a physiological change in the body and brain waves4:

Stage 1 (NREM1)

When we begin sleeping, we enter stage 1 (NREM1) where we are moving from wakefulness to unconsciousness. At this stage our eyes may open and close slowly as we drift into a light sleep where we can be easily wakened. Our body is already starting to make some physiological changes such as our brain waves slowing, our heartbeat and breathing rate slows and our muscles begin relaxing.2

Stage 2 (NREM2)

Stage 2 of Non-REM sleep (NREM2) we are still in a light sleep state – our heartbeat and breathing slows even further, our muscles relax more, our body temperature drops, and our eye movements stop. We spend most of our sleep cycle in stage 2 than in any other stage.2

Stage 3 (NREM3-4)

We are in a deep sleep when we have entered stage 3 (NREM3-4). It is difficult to wake you at this stage. our heartbeat and breathing slow to their lowest levels during sleep. Deep sleep is needed for restorative sleep so that you feel refreshed in the morning.2

After deep sleep we enter into a sleep state where there is rapid eye movement (REM).2,5 During this stage of sleep our eyes move rapidly.2,5 When we are in the sleep state REM, we may experience dreams.2,5 Our arm and leg muscles normally become temporarily paralysed during REM so we do not act out our dreams, although this may not be the case in some parasomnia such as sleep walking.2,5 Each of these sleep stages serves a function in protecting and maintaining our physical, psychological and behavioural wellbeing.2,5

We need sleep for energy metabolism

We have come a long way from the days of hunting and gathering, where we needed to save our energy for the hunt. However, our bodies still go through an energy conservation process where our metabolic rate slows by around 10%.1

Sleep affects our hunger hormones leptin (increases fulness) and ghrelin (increases appetite).1,8 When we sleep, ghrelin levels decrease because we are using less energy, so we do not need to send signals to the body to consume more food.8

There is evidence to show that a lack of sleep can increase the risk of gaining weight because sleep deprivation increases ghrelin and reduces leptin – this imbalance makes you hungrier and therefore increases our risk of obesity.1,8 Studies have shown that people who sleep less than 7 hours a day tend to gain more weight and have a higher risk of becoming obese than those who get 7 hours of slumber.6

Sleep has an important role in glucose metabolism. When we have decreased sleep quantity and/or quality there is evidence of a decrease in our sensitivity to insulin and a decrease in glucose tolerance.8 It seems that missing out on deep sleep may lead to type 2 diabetes by changing the way the body processes glucose, which the body uses for energy.6,7 This demonstrates the important role sleep has in energy metabolism and how good sleep can reduce our risk of obesity and type 2 diabetes.6

We need sleep to restore and repair

During the day, cellular components necessary for biological functions become depleted, restorative theory says that sleep is necessary for the body to repair and restore the necessary cellular components used during the day.1

Sleep has an important role in muscle repair and protein synthesis.1,9 In fact, poor sleep has been found to have profound negative consequences on skeletal muscle.9 Even one night of sleep deprivation disrupts the balance of anabolic and catabolic hormones and which results in a decrease in muscle protein synthesis.9

Sleep is important for growth. It is thought that infants and children need to sleep longer to help their cognitive and physical development.1,10 Several studies have demonstrated the positive impact sleep can have on child and infant memory, language, cognitive development and physical growth.10 The release of many important hormones for growth happen primarily during sleep.1

We need sleep for brain health

Sleep plays an important role in brain health by positively impacting our brain plasticity1 (the brain’s ability to change and adapt as a result of experience11). There is a wealth of evidence to also demonstrate the importance of sleep in memory, learning and brain waste clearance.4

Brain plasticity theory says that sleep is necessary for growth of the brain’s structure and function.1 Brain plasticity theory can be demonstrated by the extended sleep needs of infants and young children – rather than 8 hours of sleep, infants need 14 hours of sleep per day.1 Studies have shown that infants and children who sleep better at night show better motor and cognitive functioning.10

Sleep has a key role in memory retention & learning.13 Sleep quality and quantity have been closely linked in studies to student learning capacity and learning performance.14,15 Conditions like insomnia have been shown to impact cognitive functioning such as memory, attention, and concentration.15

A major function of sleep is the clearance of neurotoxic waste (toxic by-products from our brain which build up throughout the day).12 The clearance of brain waste is done by the ‘glymphatic system’ and primarily occurs during sleep. There are studies to suggest that lack of sleep and subsequent suppression of the glymphatic system may contribute to neurodegenerative disorders (e.g. Dementia, Alzheimer’s), traumatic brain injury and stroke.12,16 Therefore, good quality sleep is thought to reduce the risk of dementia.

We need sleep for heart health

When we are in a deep sleep our body goes through physiological changes that are thought to have heart protective properties such as the lowering of blood pressure and heart rate.7 In fact, a lack of deep sleep (NREM3) is associated with an increased risk of cardiovascular disease.7 Even shift work and jet lag has been shown to have a detrimental impact on heart health.7

We need sleep for a healthy immune system

A strong immune system depends on sleep.4,7,17 In fact, sleep can affect how well our body responds to infection and wound healing.4,17 When we sleep our body makes special immune cells which have an important role in fighting infection, such as cytokine, leukocytes and T-cells.17 Sleep has a particular important function when it comes to immunological memory – the ability of the immune system to quickly and specifically recognize an infection that the body has previously encountered so that it can initiate an effective immune response.17

When we don’t get enough sleep it adversely affects our immune system.17 Studies have shown that when we don’t get enough sleep we are 3 times more likely to catch a cold.18 Sleep deprivation can cause chronic inflammation and immunodeficiency which can be damaging to our health.17 Therefore, it is important to get good sleep to protect and maintain a healthy immune system.

We need sleep for good mental health

Sleep has an important role in the regulation of our emotions, and so, when we don’t get enough sleep it can be a risk factor for developing mood disorders like depression and anxiety.27 When we sleep, brain activity increases in the areas of the brain that regulate emotion20:

  • Amygdala – has a role in emotional learning & behaviour20,21
  • Striatum – main functions include motor planning, decision-making, motivation and reward22
  • Hippocampus – anterior hippocampus is involved in emotion and stress23
  • Insula – critical for human cognition (thinking) and behaviour24
  • Medial Prefontal Cortex – important for cognitive behaviour, personality expression , decision making, moderating social behaviour and speech25

A lack of sleep can impact our mental health. People with insomnia are 10 time more likely to have clinical depression and 17 times as likely to have clinical anxiety.26

Poor sleep quality can not only increase our risk of mood disorders, but it can also affect our quality of life by affecting our attention, concentration and memory and as a consequence this can impair our ability to work efficiently and safely.15

Despite the wealth of evidence to show how sleep can positively impact our energy, heart health, brain functioning, immunity and mental health – we, as a nation, are not prioritising sleep.

How can I get more sleep?

Getting a good night sleep always starts with healthy sleeping habits such as creating a comfortable sleeping environment and using techniques to relax your body and mind. However, if you are find that healthy sleeping habits (also known as sleep hygiene) are not helping you to sleep, then you may want to talk to your local pharmacist about over the counter sleeping aids.


  1. Sleep Physiology – Brinkman & Sharma
  2. Brain Basics: Understanding Sleep – National Institute of Neurological Disorders and Stroke
  3. About Sleep: Tips, Quotes and More – American Sleep Association
  4. Functions and Mechanisms of Sleep – Zielinski, Mckenna & McCarley
  5. An Overview of Sleep and Stages of Sleep – Hasan et al
  6. Why Lack of Sleep is Bad for Your Healthy – NHS
  7. Sleep: A Health Imperative – Luyster et al (Sleep Research Society)
  8. Metabolic Effects of Sleep Disruption, Links to Obesity and Diabetes – Nedeltcheva and Scheer
  9. The Effect of Acute Sleep Deprivation on Skeletal Muscle Protein Synthesis and the Hormonal Environment – Lamon et al
  10. Infant Sleep and its Relation with Cognition and Growth: A Narrative Review – Tham, Schneider & Broekman
  11. How Experience Changes Brain plasticity – Very Well Mind
  12. The Glymphatic System: A Beginner’s Guide – Jessen, Munk, Lundgaard, Nedergaard
  13. About Sleep’s Role in Memory – Rasch & Born
  14. Sleep Loss, Learning Capacity and Academic Performance – Curcio, Ferrara & Gennaro
  15. Insomnia – National Institute for Health and Care Excellence
  16. Sleep Disturbances Increase the Risk of Dementia: A Systematic Review and Meta-Analysis – Le Shi et al
  17. Sleep and Immune Function – Besedovsky, Lange & Born
  18. The Effects of Sleep Deprivation – John Hopkins Medicine
  19. Sleep and Emotion Regulation: An Organizing Integrative Review – Palmer & Alfano
  20. What is the Purpose of Sleep? – Healthline
  21. Amygdala – Britannica
  22. The Ins and Outs of the Striatum: Role in Drug Addiction – Neuroscience (Yager et al)
  23. Hippocampus – Britannica
  24. Structure and Function of the Human Insula – Uddin et al
  25. Prefrontal Cortex
  26. The Complex Relationship Between Sleep, Depression & Anxiety – Sleep Foundation
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