Sleep Explained: Stages, Functions, Disorders & Health Impact
Learn what sleep is, its stages (REM and NREM), why it is essential for brain and body health, and common sleep disorders such as insomnia and narcolepsy.
What is sleep?
A large amount of research is being conducted worldwide because, in today’s fast-paced and chaotic environment, people are sleeping less and experiencing many negative consequences. Insufficient sleep affects both mental and physical health.
Sleep is essential for a healthy body and a healthy mind. The duration and pattern of sleep strongly influence overall health, and in today’s lifestyle, getting healthy sleep has become a luxury for many people.
Sleep supports physical health, brain function, and emotional regulation, all of which are closely connected. Without adequate sleep, it becomes difficult to concentrate, think clearly, and perform daily tasks.
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Evolution of sleeping pattern
During human evolution, survival required constant protection from predators and harsh, unpredictable environmental conditions. To meet these challenges, humans evolved a specialized 24-hour sleep–wake pattern, remaining active during daylight hours for hunting and gathering and sleeping at night.
This consolidated nighttime sleep pattern helped early humans avoid predators, adapt to darkness, conserve energy, and restore physical and mental function for the following day. By organizing daily activities into two distinct phases, daytime for foraging and social activity and nighttime for rest and sleep, early humans improved their chances of survival and long-term health.
This evolutionary adaptation laid the foundation for the modern human circadian rhythm, which continues to regulate sleep, alertness, and biological processes in response to the day–night cycle.
Circadian rhythms
Circadian rhythms are natural biological patterns that help the body follow the 24-hour day–night cycle. They work together with the sleep drive, which increases the longer a person stays awake, making people feel sleepy at night and alert during the day.
These rhythms respond to environmental signals such as light and darkness. In the evening, as it gets dark, the body releases the sleep hormone melatonin, which helps prepare the body for sleep. In the morning, exposure to light stops melatonin release and helps us wake up. Changes in body temperature lower at night and higher in the morning also support sleep at night and alertness during the day.
On the basis sleeping pattern, sleeping behaviour are classified into four different types:
- Good sleep– It is regular, high-quality sleep.
- Poor sleep– It is erratic, low-quality, and relatively long.
- Long sleep– It was predictable, fairly lengthy, but of minimal quality.
- Short sleep– It was of excellent quality despite being short and irregular.
Sleep architecture
Sleep occurs in four distinct stages, moving from light sleep to deep sleep and finally a dream sleep stage, characterised by NREM and REM.
There are three non-rapid eye movement (non-REM) stages of sleep followed by rapid eye movement (REM), the final stage of sleep. Experiencing all four usually takes anywhere from 1.5 to 2 hours.
- Stage 1 NREM (Light sleep)
- Stage 2 NREM (Deeper sleep)
- Stage 3 NREM (Deepest sleep)
- Stage 4 REM (Dreaming)
Healthy individuals cycle through all four stages of sleep multiple times a night.
Stage 1 NREM Sleep (Light Sleep)
Stage 1 NREM is the first and lightest stage of non–rapid eye movement (NREM) sleep and typically lasts only a few minutes. It represents the transition from wakefulness to sleep, during which muscles begin to relax.
During this stage, brain activity, heart rate, eye movements, and breathing gradually slow down. Individuals may experience drifting thoughts or sudden muscle twitches (hypnic jerks) and can be easily awakened.
Stage 2 NREM Sleep (Deeper Sleep)
Stage 2 NREM sleep is deeper than Stage 1 and marks the onset of stable sleep. During this stage, bodily functions continue to slow, and responsiveness to external stimuli decreases, making awakening less likely than in Stage 1.
In this stage, heart rate and muscle activity further decrease, body temperature drops, and eye movements stop. Brain activity shows characteristic patterns known as sleep spindles and K-complexes. Sleep spindles are brief bursts of electrical activity that play a crucial role in memory consolidation and learning.
People spend the largest proportion of total sleep time in Stage 2, accounting for approximately 45–50% of overall sleep. This stage typically lasts about 20–25 minutes during the first sleep cycle, with its duration increasing in subsequent cycles.
Stage 3 NREM (Deepest sleep)
Stage 3 NREM sleep is the deepest stage of non–rapid eye movement (NREM) sleep and is also called slow-wave sleep or deep sleep. This stage helps a person feel refreshed and restored after waking up.
During this stage, the body carries out physical repair, including the recovery of muscles and tissues, and strengthens the immune system. Because brain and body activity are at their lowest levels, waking a person from this stage is very difficult.
Blood pressure decreases, heart rate and breathing slow down, and muscles are fully relaxed. The body also releases growth hormone, which is essential for growth and tissue repair.
Stage 3 NREM sleep accounts for about 20–25% of total sleep time, mainly occurring in the first half of the night.
Stage 4 REM (Dreaming)
REM (Rapid Eye Movement) sleep is different from NREM sleep and is the stage in which dreaming mainly occurs.
During REM sleep, brain activity becomes highly active, similar to the waking state, and rapid eye movements are observed. Although a person can be awakened more easily during this stage, waking up from REM sleep often causes grogginess or mental disorientation.
In this stage, skeletal muscles become temporarily paralyzed (a condition called muscle atonia), which prevents the body from acting out dreams. REM sleep plays an essential role in memory consolidation, learning, emotional regulation, and mental restoration.
REM sleep makes up about 20–25% of a typical night’s sleep, with longer REM periods occurring toward the morning. It usually begins around 90 minutes after a person falls asleep, and most dreams and nightmares occur during this stage.
Why good sleep is necessory
Support Mental Health
Sleep plays a vital role in maintaining overall brain and body health. It improves focus and concentration and allows the brain to register, organize, and store memories, all of which are essential for learning.
Adequate sleep helps regulate emotions and enhances a person’s ability to recognize danger and respond to threats. Healthy sleep also supports sound judgment, good decision-making, and other executive functions.
Protect Heart and Circulatory System
Studies show that sleeping less than six hours a day increases the risk of high blood pressure. Sleep helps control stress hormones in the body, so long-term lack of sleep can increase stress without a person realizing it. When you fall asleep and enter non-REM sleep, your blood pressure and heart rate slow down.
During sleep, the calming (parasympathetic) nervous system takes control, so the heart does not work as hard as it does when you are awake.
During REM sleep and when you wake up, the activating (sympathetic) nervous system becomes active, causing heart rate and blood pressure to rise to normal waking levels. A sudden increase in heart rate and blood pressure upon waking has been linked to chest pain (angina) and heart attacks.
The relationship between sleep architecture and sleeping disorder
Reduced slow-wave sleep (SWS) has been linked to mood disorders, anxiety, and insomnia. Generalized alterations in SWS are also observed in conditions such as psychosis, neurodevelopmental disorders, and anxiety disorders.
Changes in REM sleep pressure and sleep depth have been associated with disorders including anxiety, autism spectrum disorder, and schizophrenia.
In adults with depression and schizophrenia, reduced slow-wave (deep) sleep is correlated with greater symptom severity and increased rates of hospitalization.
In addition, sleep disorders such as insomnia, obstructive sleep apnea (OSA), parasomnias, narcolepsy, and restless leg syndrome are commonly associated with poor or disrupted sleep patterns.
Insomnia
Insomnia is a sleep disorder is characterized by difficulty falling asleep, staying asleep, or both. It is often linked to mental health conditions and can also predict the development of depression.
Insomnia is associated with a higher risk of cardiovascular diseases, including hypertension, ischemic heart disease, atrial fibrillation, heart attack, and stroke, as well as increased cardiovascular-related death.
Insomnia is also connected to mental health problems such as anxiety, depression, and suicidal thoughts and behaviors.
It has been linked to a higher risk of Alzheimer’s disease and is closely related to emotional dysregulation, as poor sleep affects brain regions involved in mood control.
Mental health disorders can worsen insomnia, creating a two-way relationship. Increased use of digital technology has also been shown to contribute to insomnia. In people with schizophrenia, insomnia and frequent nightmares are linked to a higher risk of suicide.
Obstructive sleep apnea
Obstructive Sleep Apnea (OSA) is a sleep disorder in which the airway becomes partially or completely blocked during sleep, leading to repeated drops in oxygen levels.
OSA is linked to mood problems and other mental health conditions. The repeated sleep interruptions caused by OSA can trigger inflammation and disrupt brain chemicals, which may contribute to psychotic symptoms.
OSA is also thought to disturb the sympathetic nervous system and the stress-related hypothalamic–pituitary–adrenal (HPA) axis.
Studies show that people with untreated OSA often have poorer emotional well-being and reduced quality of life. OSA has also been linked to an increased risk of self-harm and suicide.
Parasomnias
Parasomnias are abnormal behaviors or experiences that occur during sleep, such as unusual movements, nightmares, REM sleep behavior disorder, and sleep-related eating. These conditions are grouped together as parasomnias.
Except for REM sleep behavior disorder, which is strongly linked to Parkinson’s disease, most parasomnias are associated with anxiety, stress, and emotional disturbances.
Parasomnias are classified into REM parasomnias and NREM parasomnias based on the stage of sleep in which they occur.
Sleep paralysis is a type of REM parasomnia in which a person is temporarily unable to move or speak. It occurs during the transition between sleep and wakefulness either while falling asleep (hypnagogic) or waking up (hypnopompic).
Sleep paralysis has been linked to anxiety disorders and post-traumatic stress disorder (PTSD).
Narcolepsy
Narcolepsy is a rare neurological sleep disorder that disrupts normal sleep–wake cycles. It causes sudden and uncontrollable episodes of sleep, abnormal REM sleep, and sleep paralysis.
Narcolepsy is also associated with cataplexy, a sudden loss of muscle control triggered by strong emotions such as laughter or excitement.
People with narcolepsy often have significantly poorer mental health and emotional well-being compared to the general population.
Studies show that the impact of narcolepsy on mental health–related quality of life can be greater than that seen in chronic conditions such as epilepsy, multiple sclerosis, diabetes, and hypertension.
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References:
Ramar, Kannan, et al. “Sleep is essential to health: an American Academy of Sleep Medicine position statement.” Journal of Clinical Sleep Medicine 17.10 (2021): 2115-2119.
Howarth, Nathan E., and Michelle A. Miller. “Sleep, sleep disorders, and mental health: a narrative review.” Heart and Mind 8.3 (2024): 146-158.
Alruwaili, Nawaf W., et al. “The effect of nutrition and physical activity on sleep quality among adults: a scoping review.” Sleep Science and Practice 7.1 (2023): 8.
