Physiology of sleep

Overview of Sleep

Sleep is a natural state of rest for mind and body with closed eyes characterized by partial or complete loss of consciousness.

Loss of consciousness leads to decreased response to external stimuli and decreased body movements.

Depth of sleep is not constant throughout the sleeping period and it varies in different stages of sleep.

Sleep Requirement

Sleep requirement is not constant.

Average Sleep Requirement Per Day

Age group	Average sleep requirement
Newborn infants	18 to 20 hours
Growing children	12 to 14 hours
Adults	7 to 9 hours
Elders	5 to 7 hours

Physiological Changes During Sleep

During sleep, most of the body functions are reduced to basal level. Important changes in the body during sleep are given below.

1. Plasma Volume

Plasma volume decreases by about 10% during sleep.

2. Cardiovascular System

Heart Rate

During sleep, the heart rate reduces. It varies between 45 and 60 beats per minute.

Blood Pressure

Systolic pressure falls to about 90 to 110 mm Hg.
Lowest level is reached about 4th hour of sleep and remains at this level for sometime before waking up.
Then, the pressure starts rising.
If sleep is disturbed by exciting dreams, the pressure is elevated above 130 mm Hg.

3. Respiratory System

Rate and force of respiration are decreased.
Respiration becomes irregular.
Cheyne-Stokes breathing may develop.

4. Gastrointestinal Tract

Salivary secretion decreases during sleep or may be increased slightly.
Contraction of empty stomach is more vigorous.

5. Excretory System

Formation of urine decreases.
Specific gravity of urine increases.

6. Sweat Secretion

Sweat secretion increases during sleep.

7. Lacrimal Secretion

Lacrimal secretion decreases during sleep.

8. Muscle Tone

Tone in all the muscles of body except ocular muscles decreases very much during sleep. It is called sleep paralysis.

9. Reflexes

Certain reflexes, particularly knee jerk, are abolished.
Babinski sign becomes positive during sleep.
Threshold for most of the reflexes increases.
Light reflex is retained.
Eyeballs move up and down.

10. Brain

Brain is not inactive during sleep. There is a characteristic cycle of brain wave activity during sleep with irregular intervals of duration. Electrical activity in the brain varies with stages of sleep.

Types of Sleep

Sleep is of two types:

1. Non-rapid Eye Movement Sleep

NREM sleep or non-REM sleep.

2. Rapid Eye Movement Sleep

REM sleep.

1. Non-Rapid Eye Movement Sleep (NREM Sleep)

NREM sleep is a type of sleep without the movements of eyeballs. It is also called slow wave sleep.

NREM sleep does not occur in this type of sleep and it occupies about 70 to 80% of total sleeping period. NREM sleep is followed by REM sleep.

2. Rapid Eye Movement Sleep (REM Sleep)

REM sleep is the type of sleep associated with rapid conjugate movements of the eyeballs which occurs frequently.

Though eyeballs move, the sleep is very deep. So, it is also called paradoxical sleep.

It occupies about 20 to 30% of sleeping period.

Functionally, REM is very important because it plays an important role in consolidation of memory. Dreams occur during this period.

Differences Between REM and NREM Sleep

Characteristics	REM sleep	Non-REM sleep
1. Rapid eye movement	Present	Absent
2. Dreams	Occur	Do not occur
3. Muscle twitching	Present	Absent
4. Heart rate	Fluctuating	Stable
5. Blood pressure	Fluctuating	Stable
6. Respiration	Fluctuating	Stable
7. Body temperature	Fluctuating	Stable
8. Neurotransmitter involved	Noradrenaline	Serotonin

Stages of Sleep and EEG Pattern

Generally, every four-eyed person passes through five stages of sleep, i.e., four stages of NREM sleep followed by REM sleep.

All the stages occur periodically in cycles. One complete sleep cycle takes about 90 to 110 minutes.

Usually, adults have 5 or 6 sleep cycles every night.

Pattern of electroencephalogram (EEG) alters in each stage.

Stage 1: Stage of Drowsiness

Features

While feeling asleep and after going to bed, the person is in period of wakefulness, i.e., while lying down, the eyes are closed and mind is relaxed.
Then, the person proceeds to drowsy state.
During drowsiness, the person can be awakened easily.
Muscular activity slows down.
Some persons may have feeling of falling followed by sudden muscular contractions.
This period lasts for about 1 to 10 minutes.

EEG Pattern in Stage 1

During the stage of wakefulness, i.e. while lying down with closed eyes and relaxed mind, the alpha waves of EEG appear.

Stage 1 of sleep is characterized by diminishing alpha waves and emerging theta wave activity in brain.

Stage 2: Stage of Light Sleep

When the person goes to light sleep from drowsiness, heart rate slows down and body temperature starts decreasing.
Body prepares to go to deep sleep.
This period lasts up to 20 minutes.

EEG Pattern in Stage 2

Theta wave continues in this stage also.

In addition, theta waves are intermittently superimposed by sleep spindles or spindle bursts at a frequency of 14 per second.

Sleep spindle is produced by electrical activity in thalamus and corticothalamic fibers.

During this stage of sleep, another type of wave called K complex also appears.

It is a slow and large wave that is produced by a reaction to external stimuli while sleep.

Stage 3: Initial Stage of Deep Sleep

Stage 3 is a transitional period between light sleep and deep sleep. Hence, it is considered as initial stage of deep sleep.

EEG Pattern in Stage 3

Spindle bursts and K complex disappear.
Slow delta waves with high amplitude appear.
Frequency decreases to 1 or 2 per second.
Amplitude increases to about 100 µV.

Stage 4: Stage of Deep Sleep

Earlier, stages 3 and 4 were separated. But nowadays these two stages are combined together.

When the person enters deep sleep, it is difficult to wake up.

If someone wakes him or her up, there may be a feeling of disorientation for few minutes.

In spite of having sleep disturbances such as noise, some persons may sleep without any reaction.

This stage helps rejuvenation of the body.

There is release of many vital hormones which induce growth and development.
Immune system is boosted.
Muscles and tissues are repaired.
Body builds up energy for next day.

During this stage, children may have bedwetting, nightmares and sleepwalking. Even some adults may experience nightmares and sleepwalking.

In most of the people, stage 3 and stage 4 together last for about 40 to 60 minutes.

EEG Pattern in Stage 4

During this stage, delta waves become more prominent with low frequency and high amplitude.

REM Sleep

Often, this is considered as stage 5 of sleep.

During REM sleep, eyes move rapidly in all direction.
Dreams occur.
Muscles are derelaxed or temporarily paralyzed.
There is increase in heart rate and blood pressure.
Respiration becomes rapid, shallow and irregular.

REM sleep during the first cycle of sleep lasts only for about 10 minutes.

The duration gradually increases in successive cycles and in final cycle it may last for about 1 hour.

EEG Pattern in REM Sleep

During REM sleep, EEG shows irregular waves with high frequency and low amplitude. These waves are called desynchronized waves.

Mechanism of Sleep

Sleep occurs due to the activity of some sleep-inducing centers in brain called sleep-inducing centers.

Stimulation of the brain centers induces sleep.

Damage of the centers results in sleeplessness or persistent wakefulness called insomnia.

Sleep Centers

Complex pathways between reticular formation of brainstem, diencephalon and cerebral cortex are involved in the onset and maintenance of sleep.

However, two centers which induce sleep are located in brainstem.

Brainstem Centers Which Induce Sleep

Raphe nucleus
Locus coeruleus

Role of Raphe Nucleus

Raphe nucleus is situated in reticular formation of lower pons and medulla.

Activation of this nucleus results in NREM sleep.

It is due to release of serotonin by the nerve fibers arising from this nucleus.

Serotonin induces NREM sleep.

Role of Locus Coeruleus

Locus coeruleus is situated in reticular formation of pons.

Activation of this center produces REM sleep.

Noradrenaline released by the nerve fibers arising from locus coeruleus induces REM sleep.

Inhibition of Ascending Reticular Activating System

Ascending reticular activating system (ARAS) is responsible for wakefulness because of its afferent and efferent connections with cerebral cortex.

Inhibition of ARAS induces sleep.

Lesion of ARAS leads to coma.

Theories of Sleep

Many theories are proposed to explain “Why human beings and animals sleep?” But no single theory explains all facts about sleep completely.

Commonly accepted theories are briefed.

1. Repair and Restoration Theory

According to this theory, sleep is essential for revitalizing and restoration of physiological processes that keeps the body and mind healthy and functioning properly.

This theory states that sleep helps to restore what is lost in our body during the period of wakefulness.

This theory also suggests that NREM sleep is necessary for repairing the damaged tissues for restoring physiological functions and REM sleep for restoring mental functions.

2. Information Consolidation Theory

This theory suggests that people sleep in order to process all the information that were acquired during the whole day.

This theory also states that sleep allows the brain to prepare itself for the day to come.

3. Evolutionary Theory

This theory is also called adaptive theory or inactivity theory.

According to this theory, all species are adapted to sleep during the periods of inactivity or adapted to period when wakefulness would be dangerous.

4. Clean Up Theory

Clean up theory suggests that sleep allows the brain to clean itself off toxins and waste products produced during the period of wakefulness.

Many waste products are produced in brain tissues during normal activities during day time.

Such waste products are removed from brain tissues due to increase in flow of fluid during the periods of sleep.

5. Brain Plasticity Theory

According to brain plasticity theory, sleep is essential for neural regeneration and growth of brain tissues particularly in infants who sleep for up to 14 hours each day.

This theory also states that sleep is essential for normal functioning of brain.

It is evident by the fact the sleep deprivation affects people’s ability to learn and perform many tasks.

Applied Physiology: Common Sleep Disorders

1. Insomnia

Insomnia is the inability to sleep or abnormal wakefulness.

It occurs due to systemic illness or mental conditions such as psychiatric problems, alcoholic addiction and drug addiction.

2. Hypersomnia

Hypersomnia is the excess sleep or excess need to sleep.

It occurs because of lesion in floor of the third ventricle, brain tumors, encephalitis, chronic bronchitis and disease of muscles.

Hypersomnia also occurs in endocrine disorders such as myxedema and diabetes insipidus.

3. Narcolepsy and Cataplexy

Narcolepsy is a sudden attack of uncontrollable sleep.

Cataplexy is sudden outburst of emotion.

Both the diseases are due to hypothalamic disorders.

4. Sleep Apnea Syndrome

Sleep apnea is the temporary stoppage of breathing repeatedly during sleep.

Sleep apnea syndrome is the disorder that involves fluctuations in the rate and force of respiration during REM sleep with short apneic episode.

Sleep apnea syndrome occurs in obesity, myxedema, enlargement of tonsil and lesion in brainstem.

Common Features

Loud snoring
Restless movements
Nocturnal insomnia
Daytime sleepiness
Morning headache
Fatigue

In severe conditions, hypertension, right heart failure and stroke occur.

5. Nightmare

Nightmare is a condition during sleep that is characterized by a sense of extreme uneasiness or discomfort or by frightful dreams.

Discomfort is felt as of some heavy weight on the stomach or chest or as uncontrolled movement by the body.

After a period of extreme anxiety, the subject wakes with a troubled state of mind.

Nightmare occurs due to improper food intake, digestive disorders or nervous disorders.

It also occurs during drug withdrawal or alcohol withdrawal.

6. Night Terror

Night terror or sleep terror is a disorder similar to nightmare.

It is common in children.

The child awakes screaming in a state of fright and semiconsciousness.

Child cannot recollect the attack in the morning.

7. Somnambulism

Somnambulism or sleep walking is getting up from bed and walking in the state of sleep.

The episode lasts for few minutes to half an hour.

8. Nocturnal Micturition

Nocturnal micturition or enuresis is the involuntary voiding of urine at bed.

It is also called bedwetting.

It is common in children.

9. Movement Disorders During Sleep

Movement disorders occur immediately after falling asleep.

Sleep start or hypnic jerk is the common movement disorder during sleep.

It is characterized by sudden jerks of arms or legs.

Sleep start is a physiological form of clonus.

Other movement disorders are teeth grinding (bruxism), banging the head and restless movement of arms or legs.

Sleep Study: Polysomnography

Sleep study, also known as polysomnography, is a diagnostic test to assess various body functions during sleep.

Results of the study help diagnose sleep disorders.

During the study, sensor devices are fixed on the skin over different areas of the body such as chest, legs, scalp and temples.

All the sensors are connected to a computer.

The computer will monitor various activities of the body during sleep.

Commonly Observed Parameters During Polysomnography

Heart rate.
Respiratory pattern.
Oxygen saturation in blood.
ECG waves.
EEG waves.
Eye movements.
Body position.
Body movements such as:
1. Limb movements.
2. Chest movements.
3. Abdominal movements.
Snoring and other noises.

Uses of Polysomnography

Polysomnography is useful to diagnose sleep disorders.