What is sleep?

Sleep is a condition of reduced muscle movement in the body. Awareness about the surroundings is reduced at the time of sleep. The body stays mildly responsive or non-responsive to external stimuli. The sleep-wake cycle is controlled by the circadian rhythm that produces a resting phase for the body.

Major phases of sleep

The two different phases of the sleeping pattern are rapid eye movement sleep (REM sleep) and non-rapid eye movement sleep (NREM sleep). Both these phases form the sleep cycle and maintain circadian rhythm. They are characterized by distinct physiological traits that differentiate the two

REM sleep

REM sleep is characterized by more active brain functions. This is the phase of sleep where muscle movement is very low. This generally occurs after 90 minutes of the start of sleep. Higher brain activity helps in the detection of external stimuli while sleeping. This type of sleep is considered paradoxical, as body physiology and brain functions show similarities with the waking period.

Electroencephalogram detection shows a high level of activity in the brain. This is considered as the time when we dream. Due to those dreams, the vital signs of the individual change. REM sleep, in a phrase, can be described as an active brain in an inactive body.

Non-REM sleep

Non-REM sleep or slow-wave sleep shows reduced brain activity with low-frequency and low-voltage brain waves. It is the specific sleep stage for deep sleep that goes on for a longer period. The body repairs and regrows' tissues, builds bone and muscle and strengthens the immune system during deep sleep in NREM. As one gets older, one sleeps more lightly and gets less deep sleep.

Out of all the sleeping hours, about 1-3 hours of deep sleep is enough. As people get enough rest, REM sleep pattern stops and the NREM sleeping stages begin. NREM sleep is divided into four separate stages (Stages 1, 2, 3, and 4) of its own based on the depth of sleep.

Different stages of sleep

Circadian rhythm

The body has an internal clock that runs in the background that helps to carry out essential functions and processes. These 24-hour cycles, also known as circadian rhythms, are part of the body’s internal clock. The sleep-wake cycle is one of the most important and well-known circadian rhythms. Different systems of our body follow circadian rhythms. The rhythm of this master clock in the brain is influenced by environmental signals such as light. Therefore, we can say that circadian rhythms are influenced by the cycle of day and night.

The properly aligned circadian rhythm promotes consistent and restorative sleep. Whereas, an off circadian rhythm creates significant sleeping problems, including insomnia. Circadian rhythms play an integral role in physical and mental health of a human being. Melatonin is a hormone responsible for a body to go to sleep, and our body releases more of it at night and suppresses it during the day. On the other hand, cortisol is a hormone that makes us alert. Our body produces more of it in the morning. Also, serotonin is the wake-promoting hormone that helps people to stay awake.

Specific needs of a proper sleep

A good sleep at night is crucial for the overall health of our body. Our body undergoes various physiological, mental, and biochemical changes during a good sleep. These changes are responsible for the better functioning of our body.

Changes in neurological functions

Repairing the neural damages and slow formation of new neurons occur during sleep. New neural networks are formed during sleep to generate connections. These are not formed while we are awake. The prefrontal cortex stays dormant throughout sleep, and the cerebral cortex only stays active during REM sleep. New cortical coordination is created in such regions of the brain.

Changes in physiological functions

Wakefulness is the time when the body is constantly in function. This leads to tiredness, and humans require sleep to restore their energy and recover from work. A higher rate of cell division occurs while sleeping, more specifically during NREM sleep. It restores the body and helps the body to recover from the loss of energy so that the body is prepared for the next day of work. It also helps to restore the immune system of our body.

Changes in biochemical functions

Major changes that occur while sleeping as compared to wakefulness are the changes in metabolic rate. A sharp decrease in metabolic functions occurs while sleeping so that harnessed energy from food remains for the next day. As less heat is generated, the body temperature also falls. Body functions mainly depend on the utilization of glucose. All storage nutrients get depleted and that needs to be restored. The resting phase while sleeping is the perfect time for restoring the depleted nutrients. This helps with the next wake cycle of the individual.

Psychological changes

All the sleep stages are important for the manifestation of a proper sleep cycle. Both slow-wave sleep and deep sleep are important for maintaining memory functions. Sleep deprivation leads to a drastic impact on cognitive skills. It stops people from recalling previous information and leads to confusion, dizziness, and lack of judgment.

A proper sleep pattern is ideal for promoting wakefulness by sending arousal signals to the cerebral cortex, the biggest region of the brain. Active cortex means active person. Alertness is responsible for better cognitive abilities, problem-solving skills, and a creative mindset. Getting enough sleep is a basic requirement for mental stability and response.

Sleep deprivation disorders and complications

Proper sleep duration is a necessity for normal body functions. A proper circadian rhythm controls the sleep-wake cycle along with REM sleep and non-REM sleep. The recovery process of the body is unable to continue without sleep. Approximately being awake for 24 hours creates intense stress leading to some major sleep disorders. Some of the common disorders are narcolepsy, insomnia, Restless Legs Syndrome, and Sleep Apnea.


Sleep quality and sleep duration vary among people depending on their lifestyle. If a person is sleep-deprived for a prolonged period, they may start developing a state of insomnia. When the circadian rhythm is disrupted, it leads to individuals feeling sleepy during the day. It takes a toll on the body functions like the sleep time of the person changes.


Drowsiness during the day and having sudden episodes of falling asleep are common characters of narcolepsy. This condition occurs as a result of sleep quality. It causes sleep-wake disruption altering the daily routine of the people. Body physiology is altered from homeostasis and this also creates an imbalance in the immune system

Sleep Apnea

Sleep apnea is a serious medical condition in which breathing repeatedly stops and starts. This may lead to choking or obstruction in the air passage. Among all the sleep disorders, this has the highest risk and may cause death. Excessive weight or obesity is the root cause of this disorder. It should be treated with priority. It is also responsible for changing blood pressure, respiratory rate, and body temperature while sleeping and could be fatal.

Restless Legs Syndrome

Restless Legs Syndrome is a unique type of condition where people feel the need to move their legs. It is just because their legs at the resting state seem uncomfortable. It mainly occurs in a sitting or lying down posture because the legs are at rest. But the situation turns out to be worse with growing ages and alters the sleeping pattern.

Recent studies on sleep deprivation affecting normal health

Scientific studies on sleep deprivation and circadian rhythm disruption are conducted all over the world. It is performed to judge the effects on mental health due to reduced REM sleep and slow-wave sleep.

  • Dr. Charles Czeisker (Czeisler CA), the doctor of Sleep Medicine at Harvard, conducted numerous research on sleep deprivation and sleep-dependent control of body functions. Each of the studies suggested low cognitive abilities and problem-solving skills compared to the normal sleeping pattern.
  • Obstructive sleep apnea (OSA) is caused due to neglecting circadian disruption and sleep deprivation disorder. Promoting a better way to maintain sleep physiology directly affects the immune response as well. The fatal state only occurs by neglecting OSA that creates alternation in the blood pressure.

Context and Applications

This is a pretty unique and specific field that many students choose for research. This subject is essential for the students who are studying the specific fields of biology like:

  • Neuroscience
  • Physiology
  • Sleep Medicine
  • Psychology

Practice Problems

  1. What can be the condition that leads to a heart attack while sleeping?
    1. Insomnia
    2. Paranoia
    3. Sleep Apnea
    4. RLS

Answer-c-Sleep Apnea

2. Which hormone is responsible for the sensation of sleep?

    1. Cortisol
    2. Serotonin
    3. Melanin
    4. Melatonin


  1. During which stage of sleep our brain remains active and aware?
    1. NREM stage 1
    2. NREM stage 3
    3. REM 
    4. None of the above

Answer c-REM

  1. What happens when a person suffers from sleep deprivation?
    1. They have no control over falling asleep, even while working.
    2. Most people lose thinking and creative abilities with prolonged sleeplessness.
    3. People suffer from sudden anger issues and major stress.
    4. All of the above

Answer-d-All of the above

5. Obstructive sleep apnea mainly happens-

    1. Due to neglecting sleep deprivation
    2. Hampering circadian rhythm
    3. Secretion of less melatonin
    4. All of the above

Answer-a- Due to neglecting sleep deprivation

Common Pitfalls

It is a common mistake to think that sleep disorders are not related to circadian disruption. Circadian rhythm is not just about the sleep-wake cycle governed by external stimuli of day and night. Major changes take place in hormone secretion and physiology. Sleep duration helps with the regeneration of cells inside the system which eventually helps the individual during wakefulness.

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