What is Sleep? An Overview

Sleep is the natural intermittent state of rest for the mind and body with closed eyes characterized by fractional or total loss of consciousness. It is a usual, reversible, recurring state of decreased responsiveness to external stimulation accompanied by multifaceted and foreseeable variations in physiology. These variations are synchronized, impulsive, and internally generated brain activity, changes in hormone levels, and relaxation of musculature. A healthy, young adult usually requires about 7 to 9 hours of sleep for optimal health(1).

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Although sleep contrasts with wakefulness – a state in which there is a heightened potential for sensitivity and efficient receptiveness to external stimuli – it still includes active brain patterns, thus making it more responsive than a coma or other states of consciousness. 

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Physiological Changes During Sleep

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The reason the purpose of sleep remains uncertain is partially because sleep is an advanced state that impacts all physiology, rather than a single organ or other isolated physical systems. While you sleep, most body functions return to the basal level. The following significant changes occur in the body during sleep:

Heart Rate and Blood Pressure: The heart rate is reduced to about 45 to 60 beats per minute and the systolic blood pressure falls to about 90 to 110 mm Hg. The lowest level is reached at about the 4th hour of sleep and it stays at this level till a short time before waking up. Then, the pressure begins to rise. If the sleep is disturbed by dreams the pressure may rise to about 130mm Hg. 

Respiration: The rate of respiration decreases. 

Gastrointestinal Tract: The secretion of saliva is decreased and stomach contractions become more aggressive. 

Urine Formation: Urine formation is decreased in the kidneys and the specific gravity of urine increases.

Sweat and Lacriminal Secretion: The secretion of sweat by the sweat glands is increased while the secretion of lacrimal fluid by the lacrimal glands is decreased. 

Muscle Tone: The tone in all the muscles of the body excluding ocular muscles decreases significantly during sleep. 

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Reflexes: The threshold for most of the body reflexes increases. Light reflex is retained and eyeballs move up and down. 

Brain: The brain is not inactive during sleep.There is a typical cycle of brain wave activity during sleep broken up by dreams. The electrical activity in the brain fluctuates with stages of sleep and can be seen with an electroencephalogram (EEG).  

Endocrine: The secretion of growth hormone usually occurs during the first few hours after sleep onset and mostly takes place during slow-wave sleep, whereas the secretion of thyroid hormone occurs in the late evening. 

Sleep Cycle

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An individual’s sleep is not uniform throughout the night. Instead, sleep consists of several rounds of the sleep cycle, each of which comprises two types of sleep which are further divided into stages. On a regular night, an individual goes through four to six such sleep cycles (2). Not all of these are of the same duration, but on average they last approximately 90 minutes each.

It is typical for sleep cycles to undergo variation.The first sleep cycle is frequently the shortest. It varies from 70 to 100 minutes, with the following cycles ranging between 90 and 120 minutes. Additionally, the composition of each cycle i.e., how much time is spent in each sleep type and each sleep stage, changes as the night progresses.

Sleep cycles can differ from individual to individual and from night to night depending upon a wide variety of factors such as age, alcohol consumption, and recent sleep patterns.

​​Types of Sleep Cycle 

Sleep is divided into type types: 

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1. REM sleep or Rapid eye movement sleep

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2. NREM sleep, non-REM sleep, or non-rapid eye movement sleep  

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A sleep cycle begins with a brief period of NREM stage 1 continuing through stage 2, followed by stages 3 and 4, and lastly to REM. However, individuals do not remain in REM sleep for the remainder of the night. Instead, over the progression of sleep, NREM and REM sleep alternate cyclically. The purpose of repetitions between these two types of sleep is not yet understood. However, irregular cycling and/or absent sleep stages are linked to sleep disorders.

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NREM Sleep

Non-rapid eye movement (NREM) sleep is the type of sleep in which there is no movement of the eyeballs, also known as slow-wave sleep. It encompasses about 70% to 80% of the total sleeping duration. Non-REM sleep is followed by REM sleep, or Rapid Eye Movement sleep.NREM sleep is relaxing and is accompanied by decreases in both peripheral vascular tone and other vegetative functions of the body. For example, blood pressure, respiratory rate, and basal metabolic rate can decrease by up to 10 to 30 percent.

Although NREM sleep is often called dreamless sleep, dreams do happen during this stage. The distinction between the dreams that occur in NREM sleep and those that occur in REM sleep is that those in REM sleep are linked with more muscle activity. Additionally, the dreams of NREM sleep are typically not recalled because consolidation of dreams in memory does not typically occur. 

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Stages of NREM Sleep

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NREM sleep is divided into four stages, based on the pattern of electroencephalograms, or EEG (3). Alert wakefulness is characterized by the presence of high-frequency beta waves, whereas during the stage of quiet wakefulness, i.e., lying down with closed eyes and a calm mind, the alpha waves appear.  When the person enters a drowsy state,  the alpha waves diminish and delta waves start to appear. As the sleep episode advances, stage 2 begins to account for a major part of NREM sleep, and stages 3 and 4 may disappear altogether.

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Stage I: Stage of Drowsiness

Stage 1 sleep serves an interim role in sleep-stage cycling. In normal individuals, the sleep episode begins in NREM stage 1. The individual has not fully relaxed, though the body and brain activities begin to slow with periods of short-lived movements. It typically lasts 1 to 7 minutes in the initial cycle and constitutes about 2 to 5 percent of the total sleep duration. 

An individual in this stage of sleep is easily awoken by weak stimuli such as disruptive noises. However, if an individual is not bothered, they can quickly transition into stage 2. As the night progresses, an undisturbed individual will not spend much time in stage 1 as they move throughout the following sleep cycles. 

Brain activity on the EEG in stage 1 shows a shift from wakefulness (marked by periodic alpha waves with a frequency of 8 to 13 cycles per second) to low-voltage, mixed-frequency, infrequent delta waves. Thus, the alpha waves are reduced and the EEG shows only low voltage fluctuations and occasional delta waves. 

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Stage II: Stage of Light Sleep

During stage 2 sleep the body transitions into a more subdued state consisting of a drop in temperature, slowed breathing and heart rate, and relaxed muscles. It continues for about 10 to 25 minutes in the initial cycle and increases in duration with each successive cycle, ultimately constituting about 45 to 55 percent of the total sleep duration. An individual in this stage of sleep needs a more intense stimulus than in stage 1 to awaken.

Brain activity on an EEG is characterized by spindle bursts at a frequency of 14 per second, superimposed by low-voltage, mixed-frequency delta waves, and by the presence of k-complexes (4). It is theorized that sleep spindles are important for the consolidation of memory. Individuals who acquire a new skill or task have a significantly greater density of sleep spindles than those who haven’t (5).

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Stage III: Stage of Medium Sleep

Stage 3 persists for only a few minutes and makes up about 3 to 8 percent of the total sleep duration. During this stage, EEG shows increased high-voltage, slow-wave activity i.e., the spindle bursts fade and the frequency of waves is reduced to about 1 or 2 per second while the amplitude increases to about 100 µV.

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Stage IV: Stage of Deep Sleep

This is the last stage of NREM sleep. It lasts for about 20 to 40 minutes in the first cycle and constitutes about 10 to 15 percent of the total sleep duration. As the night progresses the duration of this stage decreases and it may even become absent altogether. In this stage, the threshold for being awakened is the highest among all stages of NREM sleep. Pulse, breathing rate, and muscle tone decrease as the body relaxes even further.

It is believed that this stage is essential for restorative sleep, enabling bodily recovery and growth. It may also strengthen the immune system and other vital bodily processes. Although brain activity is decreased, there is evidence that deep sleep plays an important role in promoting insightful thinking (6), creativity (7), and memory.

The EEG is characterized by slow wave activity, or in other words, the presence of prominent delta waves with increased amounts of high-voltage, high amplitude, and low-frequency waves. Due to this presence of delta waves, it is also known as delta sleep or slow wave sleep.

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REM Sleep

REM sleep is the type of sleep that involves rapid conjugate movements of the eyeballs, which is how this stage received its name. During REM sleep, the body experiences atonia,a momentary paralysis of the muscles, with two exceptions, the eyes and the muscles that regulate breathing. This signifies a strong inhibition of the spinal muscle control areas. Although the eyes are closed, they can be seen moving rapidly. 

Though the eyeballs move, the individual is in deep sleep and it is even more difficult to arouse them with stimuli than during deep slow-wave sleep. It comprises about 20% to 30% of the sleeping duration. Under normal conditions, an individual does not enter REM sleep until about 90 minutes have passed since they started sleeping. When a person is very sleepy, each cycle of REM sleep is brief and may even be absent. As the person becomes more relaxed through the night, the lengths of the REM cycles increase and in normal adults, it is longest in the last one-third of the sleep episode. Although the first REM stage may last only a few minutes, the following stages can last for around an hour. 

The brain is very active in REM sleep, and the total brain metabolism may be amplified by as much as 20 percent. An electroencephalogram (EEG) reveals a pattern of brain waves comparable to those that occur during wakefulness i.e., irregular desynchronized waves with high frequency and low amplitude. This type of sleep is known as paradoxical sleep because it is a “paradox” that an individual can still be asleep despite the occurrence of obvious activity in the brain. 

REM sleep is very important functionally, as it plays a vital role in cognitive functions like memory consolidation (8), learning, and creativity (9). Most vivid dreams occur during this period which is clarified by the substantial increase in brain activity. The heart rate and respiratory rate usually become irregular, which is typical of the dream state. Dreams can happen in any sleep stage, but they are less frequent and intense in the NREM stages.

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Importance of Sleep

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During sleep, the body executes several repairing and maintaining processes that involve nearly all parts of the body. Thus, a good night’s sleep, or a lack of sleep, can influence the body both mentally and physically. Some important benefits of sleep include an improvement in mood, a healthy heart, regulated blood sugar, improved mental function, and stress relief.

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Improved Mood: Good sleep helps to restore the body and boost energy levels, thus waking up well-rested can have an encouraging influence on an individual’s mood (10). A continuing lack of sleep can cause anxiety, irritability, and depression.

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Healthy Heart: Quality sleep improves heart health because while you sleep the heart rate slows down and the blood pressure decreases, therefore, allowing the heart and vascular system to rest.  A lack of sleep causes the blood pressure to remain elevated for a long period, thus increasing the risk of heart disease (11).

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Regulated Blood Sugar: Sleep influences the body’s relationship with insulin, which causes glucose, or blood sugar, to enter the cells.  The cells then use the inducted glucose as a source of energy. Having good sleep ensures that the blood sugar is regulated in the body.

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Improved Mental Function: Quality sleep plays a role in the consolidation of memory and cognitive thinking. Sleep is essential for the brain to grow, make new neural connections, reorganize, and restructure (12). A night of good sleep leads to enhanced concentration and improved problem-solving and decision-making skills, thereby increasing an individual’s productivity.

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Stress Relief: A good quality sleep each night helps to control stress (13). Waking up refreshed and well-rested helps individuals to avoid the stressors that arise from sleep deprivation, like lack of energy, poor performance, or difficulty thinking clearly. Good sleep helps to decrease depression, anxiety, and other mental health tensions linked to stress.

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Sleep Quality

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Sleep quality is a broad conceptualization that can be described from different angles. It is not only associated with the number of hours spent sleeping per night but also with how effectively an individual’s body can complete its sleep cycle. On average, more than one-third of U.S. adults sleep less than seven hours per night.Good sleep quality is assessed by how fast an individual falls asleep (the ideal time is 15-20 minutes), the ability to stay asleep, and spending most of your time in bed asleep instead of awake.

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The most vital parts of the sleep cycle are when the brain stays in a light slumber i.e., REM sleep, and when you enter deep sleep i.e., slow-wave or delta sleep. As mentioned above REM sleep plays a role in cognitive function whereas during deep sleep the body recovers and restores itself, secreting growth hormones that cause muscle gain or fat loss. The brain restores its neurotransmitter levels during this time, including serotonin which regulates mood.

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Factors Affecting Sleep Quality

Many factors influence sleep quality. These include age, stress, caffeine intake, alcohol consumption, medical conditions, environment, etc.

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Age: It has been seen that sleep problems are more prevalent in the elderly than younger adults as older individuals suffer from different age-related diseases or chronic pain, which makes it hard for them to get sufficient rest.

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Stress: Stress is responsible for a multitude of health problems, including trouble sleeping. This may be caused by the fact that excessive stress hinders the body's ability to regulate hormones and preserve mental stability.

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Caffeine Intake: Excessive caffeine intake in the form of tea or coffee can lead to sleep problems as it encourages alertness by inhibiting chemicals in the brain that are responsible for promoting sleep and relaxation. It makes it harder for an individual to go to sleep and to stay asleep.

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Alcohol Consumption: Alcohol is a CNS(Central nervous system) depressant and it makes it harder to get the restorative sleep that an individual needs. Even if one falls asleep faster and stays asleep longer, the quality of one’s sleep may still be poor because alcohol decreases the duration of rapid eye movement (REM) and deep sleep stages. Individuals who drink before going to bed usually suffer from insomnia symptoms and feel extremely sleepy the next day (14).

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Smoking Tobacco: Tobacco contains nicotine which is an addictive stimulant. It promotes the release of neurotransmitter substances in the brain and speeds up brain function, thereby affecting an individual’s sleep-wake cycle. It makes it harder to fall asleep and to stay asleep, and it hinders the body’s ability to repair itself during sleep.  

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Medical Conditions: Several medical conditions can increase a person's risk of developing insomnia. These include breathing disorders such as chronic obstructive pulmonary disease (COPD), neurological disorders, and psychiatric conditions. Also, diabetes is one of the major factors that affect sleep quality as a person has to repeatedly get up to urinate.

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Environment: Sleep quality can also be affected by environmental factors that interfere with sleep such as noise or discomfort. An individual living in a noisy environment may find it hard to fall and stay asleep. In addition, a poor-quality mattress or bed may cause an individual to toss and turn for longer in the night, making it harder to fall asleep.

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Improving Sleep Quality

If an individual is not suffering from a sleep disorder that warrants medical intervention, improving sleep quality is relatively simple. One just needs to form better sleeping habits and improve the sleeping environment.

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Making Lifestyle Changes: To improve sleep quality one needs to form good lifestyle habits such as improving diet, avoiding caffeine, alcohol, and heavy meals at least a few hours before sleep, and regular exercise. Establishing a feasible routine i.e., going to sleep and waking up at the same time every day can help improve sleep quality. Making small changes can lead to great results.

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Improving the Sleeping Environment: A relaxing sleep environment is extremely important to fall asleep faster and to maintain good sleep quality. A soothing bedroom environment should be cool i.e., at the desired temperature, quiet, and dark. There should be no distractions such as computers, mobile phones, and television at least 30 minutes to an hour before bedtime. Avoiding screen time right before going to bed helps as the blue light emitted from these devices muddles with the body’s circadian rhythms by decreasing melatonin – the sleep-inducing hormone.

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Managing Stress: Managing stress helps to significantly improve sleep quality. Activities such as yoga, meditation, and deep breathing can help to reduce stress.

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Physical Activity: Regular physical activity helps to enhance sleep quality as it helps tire the body, making it easier and quicker for one to fall asleep. Going for a walk, joining a fitness class, or doing some simple exercises in your home may help tire your muscles before you sleep. However, it is recommended to allow a few hours to pass between exercise and sleep because too much stimulation of the muscles can delay sleep.‍

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