The circadian rhythm regulates sleep, which tends to change over the course of human lives. Newborns spend about 50% of their total sleep in REM sleep, usually directly entering REM sleep. Newborns also tend to initially sleep in short intervals, obtaining around 12 to 18 hours of sleep. As children reach 5 to 10 years of age, their sleep demand decreases to 10 hours. The demand further decreases as teenagers require 8 to 9 hours and adults need 7 to 8 hours. Our circadian rhythm also controls the nocturnal release of adrenocorticotropic hormone (ACTH), prolactin, melatonin, and norepinephrine, which are all essentials hormones for normal body functioning. [2],[3]

Sleep is broken down into 5 phases: wake, N1, N2, N3, and R. Stages N1 to N3 have considered non-rapid eye movement sleep, each progressively going into deeper sleep. Sleep is staged in sequential 30-second epochs, and each of these epochs is assigned a specific sleep stage. The majority of sleep is spent in the N2 stage. [4]

Wake

The first stage is the wake stage or stage W, which further depends on whether the eyes are open or closed. During eye-open wakefulness, there are alpha and beta waves present, predominantly beta. As individuals become drowsy, and the eyes close, the alpha rhythm is the predominant pattern. An epoch is considered stage W if it contains greater than 50% alpha waves and eye movements associated with wakefulness. [5]

N1 (Stage 1)

This is the lightest stage of sleep and starts when more than 50% of the alpha waves are replaced with low-amplitude mixed-frequency (LAMF) activity. There is muscle tone present in the skeletal muscle and breathing tends to occur at a regular rate. This stage tends to last 1 to 5 minutes, consisting of around 5% of the total cycle.

N2 (Stage 2)

This stage represents deeper sleep as your heart rate and body temperate drop. It is characterized by the presence of sleep spindles, K-complexes, or both. These sleep spindles will activate the superior temporal gyri, anterior cingulate, insular cortices, and the thalamus. The K-complexes show a transition into a deeper sleep. They are single, long delta waves only lasting for a second. As deeper sleep ensues and the individual moves into N3. All of their waves will be replaced with delta waves. Stage 2 sleep lasts around 25 minutes in the initial cycle and lengthens with each successive cycle, eventually consisting of about 50% of total sleep. 

N3 (Stage 3)

This is considered the deepest stage of sleep and is characterized by a much slower frequency with high amplitude signals known as delta waves. This stage is the most difficult to awaken from, and for some people, even loud noises (over 100 decibels) will not awaken them. As people get older, they tend to spend less time in this slow, delta wave sleep and more time stage N2 sleep. Although this stage has the greatest arousal threshold, if someone is awoken during this stage, they will have a transient phase of mental fogginess. This is known as sleep inertia. Cognitive testing shows that individuals awoken during this stage tend to have mental performance moderately impaired for 30 minutes to an hour. This is the stage when the body repairs and regrows its tissues, builds bone and muscle, and strengthens the immune system.

REM Sleep

This is the stage associated with dreaming. Interestingly, the EEG is similar to an awake individual, but the skeletal muscles are atonic and without movement. The exception is the eye and diaphragmatic breathing muscles, which remain active. The breathing rate is altered though, being more erratic and irregular. This stage usually starts 90 minutes after you fall asleep, and each of your REM cycles gets longer throughout the night. The first period typically lasts 10 minutes, and the final one can last up to an hour. [6],[7]

The clinical evaluation of sleep is performed through a polysomnogram, which measures EEG (brain waves), eye movements, muscle movement of chin and legs, nasal pressure and airflow, chest and thorax movement, and pulse oximetry. These studies are consolidated to examine and assess each sleep stage for breathing and movement during sleep. These tests are performed overnight and usually require a minimum of 6 hours of monitoring.

Individuals with sleep-related disorders suffer from sleep fragmentation and apnea during sleep. As they start to fall into deeper stages of sleep, their upper airway collapses and interferes with their normal breathing. This interference forces the body to revert to the lighter sleep stages to continue better respiration. The deep sleep stages are important in normal functioning to replenish the immune system, and proper metabolism and growth. People with sleep apnea do not cycle through the normal stages of the sleep cycle. They have reduced stage N3 and REM sleep as their airway collapse prevents them from breathing in these deeper sleep states. This leads to excessive daytime drowsiness as proper, efficient sleep is not obtained throughout the night. There are two types of sleep apnea: central and obstructive. Central sleep apnea occurs when the brain fails to signal respiratory muscles during sleep properly. On the other hand, obstructive sleep apnea is a mechanical problem in which there is a partial or complete blockage of the upper airway. [8],[9]

During REM sleep, we normally do not move as our muscles are paralyzed. If the temporary paralysis (atonia) of REM sleep is disturbed, one can physically act out dreams. This is called rapid eye movement (REM) sleep disorder. With this disorder, the normal paralysis is not obtained, and there are episodes of abnormal movement throughout the night. These episodes coincide with the REM sleep stage. The cause of this disorder is not completely known but may be associated with some degenerative neurological conditions such as Parkinson's disease or Lewy body dementia. [10],[11]

Narcolepsy is another sleep cycle abnormality in which individuals usually present with persistent daytime sleepiness and brief episodes of muscle weakness known as cataplexy. In narcolepsy, the regulation of sleep is disturbed, and individuals tend to skip the initial phases of sleep and go directly into REM sleep. Even during short naps, these individuals can enter the REM phase and have short dreams. This limits their amount of sleep in the N3 deep-sleep stage and thus caused an irregular sleep pattern. These individuals also experience sudden losses of muscle strength as body muscles are atonic and paralyzed in the REM-sleep phase. These lapses into REM sleep and happen anytime during the day and usually last from seconds to minutes. [12],[13]

Somnambulism or sleepwalking is a common occurrence in school-aged children. These individuals tend to make movements that seem purposeful, and it is important to understand they are not acting out their dreams. Dreams take place during the rapid eye movement phase of the cycle, in which the body is fully paralyzed. Sleepwalking tends to occur because the sleep cycle is still in the maturing phase and proper sleep/wake cycles are not yet regulated. Sleepwalking is comply associated with common behaviors, such as dressing, eating, and urinating. Therefore, sleepwalking takes place in the non-rapid eye movement phases, usually in N3. [14]

Benzodiazepines are a major class of drugs used for the treatment of insomnia as these tend to increase the arousal threshold in stage N3 and REM sleep. These 2 stages are already known to have the highest arousal threshold and benzodiazepines further increase this threshold. They also tend to decrease the overall time spent in stage 3 and REM sleep, and thus, can be used for night time sleepwalking as these occur in the N3 sleep phase.