Why do we sleep?

Sleep constitutes an essential activity of our daily lives. Along with food and water, sleep plays an important role in restoring and maintaining the physical and mental health of an individual. It is commonly known that we spend almost one third of our lives sleeping.1

Why do we sleep

What is sleep?

Sleep is a state of unconscious body with a relatively active state of mind to its internal stimulus. Everyone needs sleep but the biological sleep is regulated by the circadian rhythm and homeostatic mechanisms of the body. The sleep-wake cycles of an individual are regulated by the various areas of the brain with coordinated hormonal release.1 This article briefly describes sleep, what happens during sleep, sleep regulating systems, why we sleep, its significance and clinical importance of sleep deprivation.

Why do we sleep?

What happens when we sleep:

Sleep is basically divided into two types: Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM) sleep. The latter is further divided into 3 stages. Each stage of sleep is associated with specific brain waves and neuronal activity. On a typical night, every individual cycles through all stages of NREM and REM several times with increasingly longer and deeper REM phases towards morning.2

The sleep-wake cycles are controlled by circadian rhythm and homeostatic mechanisms of the body. A wide variety of daily fluctuations in wakefulness to body like temperature, metabolism and release of hormones are directly regulated by circadian rhythm. They control the timing of sleep with the changing environment cues of light and temperature. The circadian rhythm is controlled roughly by 24-h biological clock and continues even in the absence of external cues of light and dark.2

On the other hand, homeostatic mechanisms of body regulate the need and intensity of sleep after certain time period. The sleep-wake cycles are influenced by several factors such as medications, associated medical conditions, stress, environment, diet, physical activity during the day and habits.2

Why do we sleep

Why do we need sleep?

The need for sleep and the pattern of sleep varies significantly with age and with individuals same age groups. Most adults including older adults need 7-8 hours of sleep every night. Despite being such an important aspect of human health, it is still unclear about the purpose of sleep. There is not one explanation about why do we need sleep to date. However, several theories have been proposed to explain the need for sleep. Some of the prominent ones are discussed below:

Energy conservation: During sleep, the function of body slows down thus reducing the energy needs to keep body systems running and conserve energy. The energy conservation theory states that the most important function of sleep is to decrease the energy needs during that part of the day or night to reduce the energy metabolism significantly. Changes in body temperature and calorie demands drop during sleep and increase while awake, support the hypothesis that sleep plays an important role to conserve energy.2

Inactivity theory: Also known as the adaptivity theory, is described as the evolutionary strategy of sleep. This theory suggests sleep as a behaviour which helps and increases the overall rate of survival. This theory hypothesized that humans evolved at a faster rate compared to other species due to their focus of getting rest. The adaptive or inactivity theory mainly suggests:2

  • All species have adapted to sleep after periods of wakefulness to reduce the risk of danger such as sleeping at night, to keep us away from the darkness when predator species take advantage of their stealth and vision.
  • It also explains sleep as an underlying physiological process similar to hunger and thrust that can be satisfied by sleeping and the urge to survive by various individuals.

Restorative theory: Another theory explaining why we need sleep is that a good night sleep is to restore and repair all the cellular components required for proper biological function lost during the active periods when we are awake. Sleep helps the body to reenergize and repair by itself. This hypothesis is supported by various physiological processes of body such as muscle repair, tissue growth, secretion of various hormones for growth that occur primarily during sleep. Other restoring aspects of sleep are specific to brain and cognitive functioning.2

Brain plasticity theory: One of the most recent and compelling theories is based on the finding that sleep is correlated to the changes that occur in the structure and function of brain. Sleep helps in neuronal reorganization and growth of brain with age. The phenomenon is not clearly understood, but the connection of sleep to the development and growth of brain in infants and children can be explained by 13-14 hours of sleep per day.2

Why do we sleep?

Functions of sleep:

Brain activity: Sleep plays an important role in the functioning of brain. Studies have shown that sufficient sleep at night helps to improve memory and learning, increase attention and creativity, and helps in making decisions. Insufficient sleep or disturbed sleep causes physical changes occuring in brain, which alter the activity and function of the brain such as decision making, problem solving, controlling emotions and coping with change.3

Memory: Sleep plays an essential role in the memory consolidation and learning. Research suggests that sleeping and dreaming helps in processing experiences and stores the gist as memories with specific details.4

Physical and hormonal health: Sleep is essential for maintenance of physical health and well being of an individual, particularly healing and repairing cells, in various systems of the body. It also helps to maintain the balance of hormones in the body, such as ghrelin and leptin, which regulate feelings of hunger and fullness and is likely to explain the link between inadequate sleep and increased risk of obesity. The insulin hormone, regulating the blood glucose, also shows variations and leading to raised blood sugar levels. For this reason, chronic sleep deficiency is also linked to a higher risk of cardiovascular disease, stroke, diabetes and kidney disease.5

Emotional health and well-being: Both negative and positive emotions play an important role and correlate with sleep. It plays an important role in processing daily stresses and emotional well-being. Negative states, such as loneliness, grief or hostility, are found to be related with increased sleep impairments. On the other hand, positive emotions, such as love, joy, happiness are associated with decreased sleep duration and improved subjective sleep quality.6

Why do we sleep

Clinical significance of sleep:

Insomnia: Insomnia is the generic term that explains difficulty falling sleep and staying sleep for various reasons. Insomnia is one of the most common sleeping disorder found in one third of the United States population. It can be acute or chronic depending upon the duration and pattern of sleep at night. Acute insomnia is usually caused by stress, some medication, travel across time zones and is generally within 3 months. On the other hand chronic insomnia is a long standing problem generally caused by various psychological, physical or behavioural factors.7 Insomnia or chronic loss of sleep usually present as trouble in concentrating or impairment in memory, malaise or fatigue, vocational or social  dysfunction, disturbances in mood or irritability, sleepiness in daytime, reduced energy and motivation, more risk of accidents, stomach issues and headache, more stress related to sleep problems.8

Narcolepsy: Narcolepsy is a chronic problem in which brain is not able to regulate the homeostatic mechanisms of wake-wake cycles. Narcolepsy is generally associated with tiredness and an uncontrollable urge to sleep during the daytime. It presents as sudden attacks of sleep at inappropriate times such as talking, driving, walking, having a meal or in a classroom or conference hall. Narcolepsy can highly disrupt the basic functions due to excessive daytime sleepiness and disrupted sleep at night.9

Obstructive sleep apnea: Sleep apnea is an involuntary cessation of breathing that happens during sleep presenting as momentary, often cyclical, rhythmic sufficient to cause significant disturbances in sleep-wake cycles and several health complications. The episodes of apnea are generally terminated by awakenings enabling the return of breathing. Sleep apnea can usually lead to sleep deprivation and constant nightly interruptions with overall shallow sleep. Reduced quality and quantity of sleep are associated with a wide range of serious health consequences that affect a person physically, mentally, and emotionally, and as a result, sleep apnea is linked to various health concerns. Due to the low oxygen levels in the body, untreated cases of sleep apnea can lead to various types of cardiac and metabolic disorders including high blood pressure, heart attack, coronary artery disease, stroke, diabetes, and mood disorders.10

Sleepwalking: Sleepwalking or somnambulism is the sudden or episodic awakening from NREM sleep and is presented by motor activity, impaired judgements, illusions, loss of memory and unresponsiveness to external stimuli. It can be caused by various factors such as stress, anxiety, medications, hereditary, sleep breathing disorders and inadequate or lack of sleep at night. Sleepwalking is usually associated with sleep deprivation with increased phases of slow-wave sleep.2

Obesity and cardiometabolic disorders: Sleep plays an important role in the regulation of blood glucose levels and recurrent reduced quantity and quality of sleep negatively impacts the carbohydrate metabolism, hormonal function leading to increasing risk of obesity and various cardiometabolic disorders. Sleep deprivation impacts the carbohydrate metabolism which is a multifactorial phenomenon including decreased utilization of glucose by brain, alterations in nervous-hormonal signaling, hormonal imbalance and inflammatory processes. The increased risk of cardiometabolic disorders due to inadequate sleep can also be explained by the increased appetite, inadequate meal timings, decreased energy expenditure and poor portion control.11


Sleep need varies from person to person and they change throughout lifetime. Sleep is as important as eating food or drinking water. While there is still no consensus why do we need to sleep, several theories explain the basic functions of sleep at the cellular level and neuronal levels. Adequate sleep and quality are important for functioning of daily metabolic, hormonal and appetite regulation. Sleeping benefits each individual in multiple ways. It has been proven that sleeping improves memory, regulates metabolism and reduces physical and mental fatigue.

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