Functions of Sleep

Functions of Sleep Overview

Sleep plays a crucial part in our lives. With enough sleep, we optimise our:

• Cognitive functioning

• Development

• Psychological state

• Immune response

• Energy conservation

Our quality of sleep directly corresponds to our quality of life. Most significantly, a healthy sleep schedule signifies a healthy brain, emotional stability, and time for the body to repair cells.

Sleep Function Theories

So why exactly do we sleep? Many psychologists have attempted to answer the question. Yet, it appears more complex than one answer, and several theories have been proposed to explain the function of sleep.

The main sleep function theories are:

• Evolution Theory

• Brain plasticity Theory

• Energy Conservation Theory

• Restoration Theory

This article will touch upon each theory whilst explaining different sleep functions and how sleep is crucial for both the brain and the body to operate.

Inactivity Theory/Evolutionary Theory

The Inactivity theory is one of the earliest sleep function theories. The theory itself indicates that the action of sleeping was an adaptation method to ensure the survival of all organisms by 'sheltering' them from vulnerability.

Whilst animals sleep, they remain quiet and still in one place, lowering their chances of being heard or seen by predators and thus sheltering them from dangerous scenarios. Essentially, this refers to sleep, a behavioural strategy that ensures survival, an essential aspect of Darwin's natural selection theory.

The Evolutionary Theory of Sleep, also known as the Adaptive theory of sleep, is based on the premise of the inactivity theory. It infers that species have adapted their sleep and periods of inactivity to a time when being 'awake' would be more threatening.

The evolutionary theory is supported by Lesku et al. (2006), who found that animals who slept in environments that put them at higher predatory risk engaged in less REM sleep.¹ REM is the stage of sleep known as rapid eye movement and is characterised by when an individual is in the deepest state of sleep.

Cognitive Function of Sleep

A regular good night's sleep is crucial for cognitive ability. If we do not get enough sleep as humans, cognitive functions such as memory, language, learning, and critical thinking may not be performed as well compared to when we have slept a healthy amount.

When people do not get enough sleep, they may have difficulties performing cognitive tasks, freepik.com

REM Sleep

Rapid Eye Movement sleep (REM) is a crucial stage in the sleep cycle. It is characterised by an increase in brain activity, rapid movements in the eyes, an increase in blood pressure and faster breathing. REM is also known to be the stage of sleep where the most dreaming occurs.

REM sleep plays a significant role in explaining the cognitive function of sleep. Research has found that REM sleep is important for memory and learning - during this stage of sleep, our brains store information that was absorbed whilst conscious.

Yet, according to certain psychologists, not all information is stored.

However, getting enough REM sleep has been proven important for performing cognitive tasks such as learning and problem-solving.

Brain Plasticity Theory

A more recent theory proposes that sleep is a time where the structure and organisation of our brain adapts and moulds to store new information.

Consider this theory in relation to infants and younger children. Known as the most crucial time for human brain development, young children require far more sleep than adults. They need this sleep for their brains to process all the new information that they learn.

The Amygdala and Sleep

When somebody is deprived of sleep, a functional deficit occurs between the anterior cingulate cortex and the amygdala, which can cause a decreased mood and increased anxiety in the amygdala (Motomura et al., 2013).⁴

Along with other brain functions such as hormone rebalance, the amygdala can perform at an optimum level with enough sleep, and individuals are more likely to experience regulated moods and emotional responses.

Role of Sleep in Physical Restoration

Sleep is a crucial time for physical repair. The restoration of cells, tissues and muscles during sleep affects all functioning systems in the human body.

Immunity and Sleep

It has been proven that bodily processes during sleep help remove harmful matter or 'junk'.

• Clearance of the brain is facilitated by the opening up of the lymphatic system at night. As the space between our brain cells expands, it leaves room for fluid to drain, removing some toxins in the brain.
• A chronic lack of sleep increases an individual's risk of developing non-infectious diseases such as cardiovascular disease.
• Sleep deprivation has also been linked to immunosuppression.

Oswald's Restoration Theory

Dr Ian Oswald proposed that sleep is necessary for lost energy to recover and restore ourselves. He suggested that sleep is crucial for the repair of cellular components.

Oswald also differentiated between the functions of REM sleep and NREM (non-rapid eye movement) sleep, also known as short-wave sleep (SWS):

• He suggested that NREM sleep is for restoring bodily functions such as physical activity, whilst REM sleep is for restoring brain functions.
• A study by Shapiro et al. (1981) provides supporting evidence and therefore increases the validity of Oswald's theory. The research found that long-distance runners had more NREM sleep after physical exertion, suggesting a link between the need for bodily restoration and NREM sleep.⁵
• Zielinski and Krueger (2013) found that a lack of NREM sleep has been linked to a lower functioning immune system, suggesting that it is important in the process of bodily restoration.

How valid is the restoration theory?

• Findings concerning the functioning of the body during sleep support Oswald's theory. It has been found that our body undergoes muscle repair, protein synthesis and the release of growth hormones during sleep.
• Sleep has been found to aid recovery from physical injury. There is an increase in blood flow during deep sleep stages; this means more oxygen and amino acids are carried to muscles to support the repair process.
• Yet, criticism of the theory stems from the question of whether the body undergoes restoration each night.