Sleep: Mechanism and Disorders

Sleep is largely determined by the body’s internal clock, which also takes into account external factors such as weather and sunlight. The natural sleep-and-wake cycle of the body is attuned fairly closely to a 24-hour period.  


Several parts of the brain are involved in sleep and wakefulness. Areas such as the thalamus, hypothalamus, basal forebrain, pineal gland, and parts of the brainstem are included in these regions. Circadian rhythm and sleep homeostasis interact to determine when a person sleeps. This circadian rhythm is used to control the body’s internal clock, and its rhythm is controlled by cells in the suprachiasmatic nucleus of the hypothalamus. Though external light and temperature affect it, the sleep cycle is roughly 24 hours long. Sleep drive/homeostasis determines how long a person stays awake and how much sleep he or she needs. Even though the exact mechanism controlling this phenomenon is not known, it is believed that it is related to the release of the neurotransmitters GABA and adenosine from the basal ganglia and other areas.  

In addition to the thalamus, basal forebrain, and brainstem that release stimulatory hormones, the hypothalamus in particular drives wakefulness with glutamate, histamine, and orexin. The hypothalamic signal is decreased when the time for sleep approaches based on the circadian rhythm and sleep drive. Through the sleep drive, the central autonomic system and pineal gland also work in coordination to appropriately affect the sympathetic and parasympathetic nervous systems, and so impact the changes in bodily functions during sleep.  


Each sleep stage has four stages; one for rapid eye movement (REM) sleep, and three for non-REM sleep (NREM). These stages are determined by analyzing patterns of brain activity that are distinctive to each stage during sleep.  

  The process of breaking down a person’s sleep into various phases and cycles is known as sleep architecture. The sleep architecture can be visually represented in a hypnogram if someone has undergone a sleep study. According to the American Academy of Sleep Medicine (AASM), the classification of sleep stages was updated in 2007. Previously, sleep experts generally referred to five stages of sleep, but today, the four AASM stages are the most accepted definition of the sleep cycle.  

Patterns of NREM sleep

The NREM sleep cycle consists of three stages. When a person sleeps in a stage called non-relaxation, waking them from sleep is more difficult.  

Stage 1 / N1

Stage 1 is basically when you doze off and lasts for one to five minutes. The body hasn’t fully relaxed by N1 sleep, but we start to see short periods of movement (twitches) as the body and brain begin to slow down. As sleep occurs, there are some changes in brain activity. During this sleep stage, it’s easy to wake a person up, but if they aren’t disturbed, they can move quickly into stage 2. An uninterrupted sleeper won’t spend much time in stage 1 as they move through the night.  

Stage 2 / N2

In stage 2, the body enters a more relaxed state with lowered body temperatures, relaxed muscles, and slower breathing and heart rates. A new wave pattern appears in the brain and eye movements cease. During sleep, brain activity slows overall, but short bursts of activity can help the brain resist being aroused by external stimuli. During the first sleep cycle, stage 2 can last from 10 to 25 minutes and N2 stages can become longer throughout the night. On average, a person spends about half their sleep time in N2 sleep.

Stage 3 / N3

During Stage 3 sleep, which is also called deep sleep, it is harder to wake someone up if they are in this phase. During N3 sleep, muscle tone, pulse, and breathing rate decrease as the body relaxes even more. This period of time has an identifiable pattern of brain activity called delta waves. Therefore, stage 3 may also be called delta sleep or short-wave sleep (SWS). This phase of sleep enables the body to recover and grow, so it is critical to restorative sleep. Additionally, it may aid in body functions such as the immune system. There is evidence that deep sleep enhances creativity, memory, and insightful thinking, even though brain activity is reduced. Deep sleep occurs most often during the first half of the night. During the early sleep cycle, the N3 stages typically last between 20 and 40 minutes. This process gets shorter as you sleep, and you spend more time in REM sleep.  

REM Sleep Patterns

When you’re asleep, your brain activity doubles, nearing levels that are seen when you’re awake. Additionally, the body has atonia, which is a temporary paralysis of muscles aside from the muscles that control breathing and the eyes. This stage is so named because even though the eyes are closed, they move rapidly. Memory, learning, and creativity are thought to be enhanced by REM sleep. A significant increase in brain activity during REM sleep explains why REM dreams are the most vivid. Even though dreams are possible in all sleep stages, they are less prevalent and less intense during NREM periods. Sleeping for 90 minutes or more usually induces REM sleep. REM sleep stages get longer over the course of the night, especially in the second half of the night. In contrast to the initial cycle of REM, which may last just a few minutes, the later stages may last up to an hour. An adult’s REM sleep accounts for about 25% of the total amount of sleep.


Sleep stages are important because they allow the brain and body to recuperate and develop. Failure to obtain enough of both deep sleep and REM sleep8 may explain some of the profound consequences of insufficient sleep on thinking10, emotions, and physical health. Sleepers who are frequently awoken during earlier stages, such as people with sleep apnea, may struggle to properly cycle into these deeper sleep stages. People with insomnia may not get enough total sleep to accumulate the needed time in each stage.


In general, sleep stages follow a given pattern, but individual variations will vary based on factors such as:  

  • Age: As a person’s life progresses, the amount of time spent in each stage changes. A newborn spends the majority of their sleep time (around 50%) in REM sleep and may enter the REM stage as soon as they fall asleep. By the time they reach 5 years of age, their sleep has become comparable to that of adults. In contrast, elderly people tend to sleep less in REM.
  • Current sleep patterns: It is possible for someone who gets insufficient or irregular sleep over a longer period to develop an abnormal sleep cycle.
  • Alcohol consumption: Some drugs and alcohol can affect sleep architecture. For example, an initial decrease in REM sleep following alcohol consumption is followed by a rebound in REM sleep with long REM stages.
  • Sleep disorders: The presence of sleep apnea, restless legs syndrome (RLS) and other conditions that wake you up multiple times can interrupt a healthy sleep cycle.  


Circadian rhythm and homeostasis are two internal biological mechanisms that regulate when you are awake and asleep.    

The Circadian Rhythm controls a wide range of functions, including sleep, body temperature, metabolism, and hormone release.  During the night, they cause us to feel sleepy and influence our tendency to wake up without an alarm.  Circadian rhythms are regulated by our body’s biological clock, which runs on a 24-hour cycle.  The circadian rhythms are synchronized with environmental cues about the actual time of day (light, temperature), but they continue even with no environmental cues.   

The sleep-wake homeostasis tracks how much sleep you need.  This sleep drive is responsible for reminding and regulating sleep intensity after a certain time.  When you are sleep-deprived, this sleep drive intensifies with every hour you are awake. This leads to deeper sleep and longer sleep.  

Aspects that affect your sleep-wake cycles needs such as medical conditions, medications, stress, sleep environments, and what you eat and drink.  One of the greatest influences is exposure to light.  A special type of cell in your retina processes light in order to tell the brain if it is day or night, which helps to regulate sleep-wake cycles.  Exposure to light can interfere with falling asleep and returning to sleep after waking up.  

Because the circadian rhythm and sleep-wake cycle are disrupted, night shift workers often have difficulty falling asleep and staying awake at work.  People suffering from jet lag experience a mismatch between their internal clock and the actual clock when they cross time zones. Their circadian rhythms become out of sync with the time of day when they fly across time zones.   


Sleep chemicals: The number of sleep-promoting neurons increases in many parts of the brain during the ante-bedtime ritual.  A chemical called a neurotransmitter can turn off or dampen activity in the cells that signal arousal or relaxation.  The chemical GABA is related to sleep, muscle relaxation, and sedation.  While we are awake, norepinephrine and orexin (also called hypocretin) keep some parts of the brain active.  Additionally, acetylcholine, histamine, adrenaline, cortisol, and serotonin are neurotransmitters that regulate sleep and wakefulness.  

Sleep and genes: The amount of sleep we need may be influenced by our genes.  Researchers have identified several genes involved in sleep and sleep disorders, as well as genes controlling neuron excitability and “clock” genes. Per, tim, and Cry which influences circadian rhythms and sleep timings.  A genome-wide association study found that sleep disorders can be linked to numerous chromosomes.  Furthermore, various genes have been linked to such sleep disorders as familial advanced sleep-phase disorder, narcolepsy, and restless legs syndrome.  There are some genes expressed in the cerebral cortex and other brain areas that change their level of expression between sleep and wakefulness.  A variety of genetic models, including worms, fruit flies, and zebrafish, are helping scientists identify the genetic components and molecular mechanisms that control normal and pathological sleep.   

Studies of sleep: To diagnose a sleep disorder, your physician may recommend a polysomnogram or other test.  Polysomnograms require spending the night at a sleep clinic or sleep lab.  Your breathing, oxygen levels, eyelid and limb movements, heart rate, and brain waves are recorded all night long.  Video and audio recordings of your sleep are also captured.  Having this data can help a sleep specialist determine if you’ve reached each sleep stage and are proceeding properly through them.  Depending on the results, a treatment plan may be developed or additional tests ordered.   


Insomnia a lack of sleep or difficulty falling asleep is reported by 20-30% of the population. Insomnia can be defined subjectively. Studies demonstrate that people can function well on as little as one hour of sleep a day. People who report sleeping little or not enough are often found to sleep much more than they thought. The reason people describe themselves as insomniacs is that they feel that they should sleep more than they do. People suffering from these conditions may show alterations in their daytime behavior and performance. Emotional arousal often results in insomnia. For example, both positive and negative thoughts about upcoming events can cause sleep disturbances. Psychologically, anxiety is often a factor in difficulty falling asleep, and depression is frequently a factor in frequent wakings  

Sleep apnea is a particular type of insomnia. . In this scenario, the sleeping person ceases breathing and is awakened when the rising amount of carbon dioxide in their blood stimulates their central chemoreceptors. The sufferer may not recall this when they are awake, and may simply report feeling tired throughout the day. There are two main causes of sleep apnea:  

  • Having a restricted airway (snorers tend to have sleep apnea);
  • The inability of the brain to initiate breathing with a normal concentration of CO2.

Sleepwalking often occurs during SW sleep, during which narrative dreams are least likely, so it isn’t what we think of as the enactment of dreams. It is most common among children, and its cause is unknown; however, it is not indicative of any underlying pathology.  

Enuresis or bed-wetting is often observed during early childhood. Most of the time, its persistence is the result of the child’s inability to wake up when the bladder is stimulated. It may also indicate anxiety-related conditions. The most common treatment is a ‘pad-and-bell’ device, in which a moisture-sensitive pad is placed underneath the sheets. An electrical circuit is completed with the first drop of urine, which results in the bell ringing and waking up the child. The child quickly learns that he or she must awaken in response to bladder stimuli before bedwetting occurs.  

Night terrors are also commonly reported in children. SW’s sleeping child wakes suddenly, terrified. This is not a nightmare (which is a very frightening dream in REM sleep). Without treatment, night terrors typically disappear with age.  

During narcolepsy, an irresistible urge to fall asleep suddenly takes over sufferers while awake. The situation may occur at any time, but it most frequently occurs during monotonous times. Typically, the sleep lasts around 5 – 10 minutes, and the person feels refreshed when they wake up. Unlike the normal sleep cycle, narcoleptics often fall into REM sleep as soon as they fall asleep.  

In cataplexy, some narcoleptics may collapse suddenly and with all muscle tone, usually fully conscious. In REM sleep, paralysis is similar to what happens normally. An animal breed with narcolepsy has been developed. There are genes associated with narcolepsy in dogs, and one gene has been found to cause it. In spite of this, genetic factors in humans are more complex. The source of the problem seems to be excessive excitability of cholinergic neurons in the peribronchial area, stimulating the cells in the magnocellular nucleus that produce the muscular paralysis of REM sleep (Nishino et al., 1994). The tricyclic antidepressants used in narcolepsy create an effect called REM sleep reduction.  

Adult sleepwalking also called REM sleep behavior disorder (RSBD) different from childhood sleepwalking. The condition is characterized by the absence of atonia during REM sleep and by dreaming abnormally, often acting out the dreams violently (Mahowald et al., 1990). RSBD may be the result of interference with the mechanism that normally produces REM sleep atonus in the magnocellular nucleus in the medulla. This situation can arise for a variety of reasons, which differ in different cases (Ferini-Strambi and Zucconi, 2000).  

As a result of flying across time zones, or working night and day shifts, our internal clocks are often out of sync with the local time. For example, our peak alertness and other functions are delayed by a minimum of six to seven hours when flying east across the Atlantic Ocean known as phase advance, in which zeitgebers are advance of the previous formed cycle. Jet lag causes to have a loss of concentration, disturbance in mood and sleep during local waking hours. In each case, the internal rhythm and the zeitgebers are not synchronized. The process of recovering from jet lag involves resynchronizing the internal and local rhythms. The westward flight (phase delay) results in less jet lag than eastward flight since we are able to adjust to it more quickly because of the shorter night during the transition. To sync the new zeitgebers we have to sleep late while going west and early in the case of the east. Getting to sleep earlier is more difficult than delaying sleep. For every hour of the time difference, an eastbound flight might require approximately one day of the adjustment. In a similar fashion, moving to a later work shift results in fewer problems than moving to an earlier shift. You can minimize jet lag by adjusting your wake-up time earlier in the morning, which helps your circadian rhythm catch up and enhances entrainment. Intense light exposure in the morning, along with intense exercise, facilitates rapid resynchronization (Boulos et al., 1995). Adjustment to phase shift can also be treated with melatonin, a hormone produced by the pineal gland(Deacon and Arendt, 1996).  


Through CBT, you can change the way you think about sleep, as well as change habits and behaviors that may keep you from sleeping well. A therapist may use the techniques mentioned below according to your specific needs and symptoms:  

SRT (Sleep Restriction Therapy) eliminates naps, thereby reducing sleep time, and makes you stay up for a longer period of time than usual. Sleep deprivation techniques are particularly effective in treating insomnia. As a result, you’re not only more tired the next day but you establish a stronger link between trying to sleep and being up late.  

Stimulus control therapy improves your sleep quality by identifying and changing bad sleep habits. The key is to train yourself to use your bedroom for nothing else than sleep and sex, and to maintain consistent sleep times, even on weekends.

Paradoxical intention is also referred to as passive awake. It is paradoxical that worrying about not being able to fall asleep keeps you awake, but letting go of this worry and making no effort to sleep may help you to unwind and sleep.

Using biofeedback specific physiological functions, such as heart rate and breathing, can be measured. It can help us recognize and control our body’s anxious response, which adversely affects our sleep patterns.  

Improve our sleep environment and sleep hygiene. The environment in which we sleep should be dark, quiet, cool, and comfortable, so our therapist may suggest blackout shades, earplugs, or a sound machine to block out all noise. For better sleep hygiene, we should teach ourselves how to exercise regularly, avoid nicotine and caffeine late at night, and unwind at night.  

Train yourself to relax. Regularly practicing mindfulness meditation, progressive muscle relaxation, and breathing exercises can reduce anxiety and tension while you sleep and aid you in your relaxation process.

Meditation with mindfulness. Try sitting quietly and noticing your natural breathing and the way you feel as you sit. Let thoughts and emotions come and go without judgment, but always return to your breathing and your body.  

Breathing abdominally is when you breathe deeply and fully, not only the chest but also the belly, lower back, and ribs can be relaxed. Put your eyes closed and take deep, slow breaths, each one becoming deeper than the previous one. Inhale through your nose and exhale through your mouth.  

Hypnotherapy is sometimes used in CBT for sleep disorders as well. Hypnotherapists use different therapeutic techniques while you are in a state of deep relaxation to assist you in changing negative behaviors and promoting restful sleep.  

The progressive relaxation of muscles. Ensure that you are comfortable. Start by tightening the muscles in your feet as much as possible. Hold for 10 seconds, then relax. Repeat this for all the muscles in your body, working your way up to the top of your head.  

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