Sleep is the natural periodic state of rest for the 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. The depth of sleep is not constant throughout the sleeping period. It varies in different stages of sleep.
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Sleep Requirement
Sleep requirement is not constant. However, the average sleep requirement per day at different age groups is:
- Newborn infants — 18–20 hours
- Growing children — 12–14 hours
- Adults — 7–9 hours
- Old persons — 5–7 hours
Read And Learn More: Medical Physiology Notes
Physiological Changes During Sleep
During sleep, most of the body functions are reduced to the basal level. Following are the important changes in the body during sleep:
1. Plasma Volume: Plasma volume decreases by about 10% during sleep.
2. Cardiovascular System:
- Heart Rate: Dun no sleep, the heart rate reduces. It varies between 45 and 80 beats per minute.
- Bio on Pressure: Systolic pressure falls to about 90-110 mm Hg. The lowest level is reached about the 4th hour of sleep and remains at this level till a short time before waking up. Then, the pressure commences to rise. If sleep is disturbed by exciting dreams, the pressure is elevated above 130 mm Hg.
3. Respiratory System: The rate and force of respiration are decreased. The respiration becomes irregular and chain-smoke type of periodic breathing may occur.
4. Gastrointestinal Tract: Salivary secretion decreases during sleep. The secretion of gastric juice is not altered or may be increased slightly. The contraction of an empty stomach is more vigorous.
5. Excretory System: The formation of urine decreases. The specific gravity of urine increases.
6. Sweat Secretion: Sweat secretion increases during sleep.
7. Lacrimal Secretion: Lacrimal secretion decreases during sleep.
8. Muscle Tone And Reflexes:
- Muscle tone reduces. Some reflexes, particularly the knee-jerk, are abolished. Babinski’s sign becomes positive during deep sleep. The threshold for most of the reflexes increases.
- Pupils are constricted. Light reflex is retained. Eyeballs move up and down. The tone in all the muscles of the body except ocular muscles decreases very much during sleep. It is called sleep paralysis.
9. Brain: The brain is not inactive during sleep. There is a characteristic cycle of brain wave activity during sleep, with irregular intervals of dreams. The electrical activity in the brain varies with stages of sleep (see below).
Types Of Sleep
The sleep is of two types:
- Rapid eye movement sleep or REM sleep
- Non-rapid eye movement sleep, NREM sleep, or non-REM sleep.
1. Rapid Eye Movement Sleep:
REM sleep is the type of sleep associated with rapid conjugate movements of the eyeballs, which occurs frequently. Though the eyeballs move, the sleep is deep. So, it is also called paradoxical sleep. It occupies about 20-30% of the sleeping period. Functionally, REM sleep is very important because it plays an important role in the consolidation of memory. Dreams occur during this period.
2. Non-Rapid Eye Movement Sleep:(Nrem Or Non-Rem Sleep):
NREM sleep is the type of sleep without the movements of the eyeballs. It is also called slow-wave sleep. Dreams do not occur in this type of sleep, and it occupies about 70-80% of the total sleeping period. The non-REM sleep is followed by REM sleep. The differences between the two types of sleep are given in.
Stages Of Sleep And EEG Pattern
1. Rapid Eye Movement Sleep: During REM sleep, the EEG shows irregular waves with high frequency and low amplitude. These waves are desynchronized waves.
2. Non-Rapid Eye Movement Sleep: NREM sleep is divided into four stages, based on the LEG pattern. During the stage of wakefulness, i.e. while down with closed eyes and relaxed mind, the alpha waves of EEG appear. When the person proceeds to a drowsy state, the alpha waves diminish.
- Stage 1: Stage of Drowsiness: Alpha waves are diminished and abolished. EEG shows only low voltage fluctuations and infrequent delta waves.
- Stage 2: Stage of Light Sleep: It is characterized by spindle bursts at a frequency of 14 per second, superimposed by low voltage delta waves.
- Stage 3: Stage of Medium Sleep: During this stage, the spindle bursts disappear. The frequency of delta waves decreases to 1 or 2 per second and amplitude increases to about 100 pV.
- State 4: Stage of Deep Sleep: Delta waves become more prominent with low frequency and high amplitude.
Mechanism Of Sleep
Sleep occurs due to the activity of some sleep-inducing centers in the brain. Stimulation of these centers induces sleep. Damage of sleep centers results in sleeplessness or persistent wakefulness called insomnia.
1. Sleep Centers
Complex pathways between the reticular formation of the brain stem, telencephalon, and cerebral cortex are involved in the onset and maintenance of sleep. However, two centers are located in the brain stem, which induce sleep:
- Raphe nucleus
- Locus ceruleus of pons.
Recently, many more areas, which induce sleep are identified in the brain of animals. Inhibition of ascending reticular activating system also results in sleep.
- Role of Raphe Nucleus: It is situated in the lower pons and medulla. Activation of this nucleus results in non-REM sleep. It is due to the release of serotonin by the nerve fibers arising from this nucleus. Serotonin induces non-REM sleep.
- Role of Locus ceruleus of Pons: Activation of this center produces REM sleep. Nor-adrenaline released by the nerve fibers arising from locus ceruleus induces REM sleep.
- Inhibition of Ascending Reticular Activating System (ARAS): The ARAS is responsible for wakefulness because of its afferent and efferent connections with the cerebral cortex. The inhibition of ARAS induces sleep. The lesion of ARAS leads to permanent somnolence, i.e. coma.
Applied Physiology – Sleep Disorders
1. Insomnia: Insomnia is the inability to sleep or abnormal wakefulness. It is the most common sleep disorder. 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 lesions in the 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 the sudden attack of uncontrollable sleep. Cataplexy is a sudden outburst of emotion. Both 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 a disorder that involves fluctuations in the rate and force of respiration during REM sleep with short apneic episodes.
- The apnea is due to decreased stimulation of respiratory centers, arrest of diaphragmatic movements, airway obstruction or a combination of all these factors. When breathing stops, the resultant hypercapnia and hypoxia stimulate respiration.
- Sleep apnea syndrome occurs in obesity, myxedema, enlargement of tonsils, and lesion in the brainstem. The common features of this syndrome are loud snoring, restless movements, nocturnal insomnia, daytime sleepiness, morning headache, and 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. The discomfort is felt as of some heavy weight on the stomach or chest or as uncontrolled movement of the body.
- After a period of extreme anxiety, the subject wakes with a troubled state of mind. It occurs mostly during REM sleep. 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 is a disorder similar to nightmares. It is common in children. It is also called pavor nocturnes or sleep terror. The child awakes screaming in a state of fright and semi-consciousness.
- The child cannot recollect the attack in the morning. Nightmare occurs shortly after falling asleep and during non-REM sleep. There is no psychological disturbance.
7. Somnambulism:
- Somnambulism is getting up from bed and walking in a state of sleep. It is also called walking during sleep or walking (Somnus = sleep; ambulance = to walk). It varies from just sitting up in bed to walking around with eyes open and performing some major complex tasks.
- The episode lasts for a few minutes to half an hour, it occurs during non-REM sleep. In children, it is associated with bed-wetting or night terror without any psychological disturbance. However, in” adults it is associated with psychoneurosis.
8. Nocturnal Enuresis: It is the involuntary voiding of urine at the bed. It is also called or bed-wetting. It is common in children.
9. Movement Disorders During Sleep:
- Movement disorders occur immediately after falling asleep. Sleep start or hypnic jerk is a common movement disorder during sleep. It is characterized by sudden jerks of arms or legs. The sleep start is a physiological form of clonus.
- The other movement disorders are teeth grinding (bruxism), banging the head, and restless moment of arms or legs.
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