Sleep is defined as unconsciousness from which the person can be aroused by sensory or other stimuli. It is to be distinguished from coma, which is unconsciousness from which the person cannot be aroused. There are multiple stages of sleep, from very light sleep to very deep sleep; sleep researchers also divide sleep into two entirely different types of sleep that have different qualities, as follows.
the cortical activation necessary to maintain wakefulness is supported by an extensive network of subcortical structures and pathways. Major neurochemicals of this “ascending arousal system” include excitatory norepinephrine arising from the locus ceruleus , serotonin from the midline raphe nuclei, histamine from the tuberomammillary nucleus, dopamine from the ventral periacqueductal gray matter, acetylcholine from the pedunculopontine tegmentum, and the laterodorsal tegmentum of the pons and orexin from the perifornical area.
TYPES OF SLEEP
Sleep is of two types:
- Rapid eye movement sleep or REM sleep
- Non-rapid eye movement sleep, NREM sleep or non-REM sleep.
RAPID EYE MOVEMENT SLEEP – REM SLEEP
Rapid eye movement 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% to 30% of sleeping period. Functionally, REM sleep is very important because, it plays an important role in consolidation of memory. Dreams occur during this period.
There are several important characteristics of REM sleep:
- It is usually associated with active dreaming and active bodily muscle movements.
- The person is even more difficult to arouse by sensory stimuli than during deep slow-wave sleep, and yet people usually awaken spontaneously in the morning during an episode of REM sleep.
- Muscle tone throughout the body is exceedingly depressed, indicating strong inhibition of the spinal muscle control areas.
- Heart rate and respiratory rate usually become irregular, which is characteristic of the dream state.
- Despite the extreme inhibition of the peripheral muscles, irregular muscle movements do occur. These are in addition to the rapid movements of the eyes.
NON-RAPID EYE MOVEMENT SLEEP – NREM OR NON-REM SLEEP
Non-rapid eye movement (NREM) sleep is the type of sleep without the movements of eyeballs. It is also called slow-wave sleep. Dreams do not occur in this type of sleep and it occupies about 70% to 80% of total sleeping period. Non-REM sleep is followed by REM sleep.
This sleep is exceedingly restful and is associated with decrease in both peripheral vascular tone and many other vegetative functions of the body. For instance, there are 10 to 30 per cent decreases in blood pressure, respiratory rate, and basal metabolic rate.
Although NON-RAPID EYE MOVEMENT SLEEP is frequently called “dreamless sleep,” dreams and sometimes even nightmares do occur during slow-wave sleep. The difference between the dreams that occur in slow-wave sleep and those that occur in REM sleep is that those of REM sleep are associated with more bodily muscle activity, and the dreams of slow-wave sleep usually are not remembered. That is, during slow-wave sleep, consolidation of the dreams in memory does not occur.
STAGES OF SLEEP AND EEG PATTERN
RAPID EYE MOVEMENT SLEEP
During REM sleep, electroencephalogram (EEG) shows irregular waves with high frequency and low amplitude. These waves are desynchronized waves.
NON-RAPID EYE MOVEMENT SLEEP
The NREM sleep is divided into four stages, based on the EEG pattern. During the stage of wakefulness, i.e. while lying down with closed eyes and relaxed mind, the alpha waves of EEG appear. When the person proceeds to drowsy state, the alpha waves diminish.
- Stage I: Stage of Drowsiness Alpha waves are diminished and abolished. EEG shows only low voltage fluctuations and infrequent delta waves.
- Stage II: Stage of Light Sleep Stage II is characterized by spindle bursts at a frequency of 14 per second, superimposed by low voltage delta waves.
- Stage III: Stage of Medium Sleep During this stage, the spindle bursts disappear. Frequency of delta waves decreases to 1 or 2 per second and amplitude increases to about 100 µV.
- State IV: 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 brain. Stimulation of these centers induces sleep. Damage of sleep centers results in sleeplessness or persistent wakefulness called insomnia.
Complex pathways between the 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:
- Raphe nucleus
- Locus ceruleus of pons.
Recently, many more areas that induce sleep are identified in the brain of animals. Inhibition of ascending reticular activating system also results in sleep.
Role of Raphe Nucleus: Raphe nucleus is situated in lower pons and medulla. Activation of this nucleus results in non-REM sleep. It is due to 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. Noradrenaline released by the nerve fibers arising from locus ceruleus 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 permanent somnolence, i.e. coma.
Ionic Regulation of Sleep
The Shaker gene is part of a voltage-gated K+ channel that regulates sleep/wakefulness, another potassium channel, Kv3.1, is present in GABAergic cells that express the Ca2+ binding protein parvalbumin, notably in fast spiking neurons.
The strongest evidence that Ca2+ channels play a role in sleep regulation is provided in studies involving (transient-type) T-type Ca2+ channels, but evidence also suggests that L-type Ca2+ channels may also modulate sleep
Endocrine Manifestations of Sleep and Wake States
hormones are little influenced by sleep versus wakefulness, including adrenocortotropic hormone, cortisol, and melatonin; some are strongly influenced by sleep, such as thyroid-stimulating hormone (TSH) and prolactin; and some are affected by particular sleep stages, such as growth hormone
The suprachiasmatic nucleus sends signals to other parts of the brain that control hormones and body temperature. Then, signals travel from the brain down the spinal cord and back up to the pineal gland, a small pinecone-shaped organ in the brain where melatonin production takes place. During the day, such signals prevent the pineal gland from producing melatonin. But when it is dark outside, these signals are not activated, and the pineal gland is able to produce melatonin
PHYSIOLOGICAL CHANGES DURING SLEEP
During sleep, most of the body functions are reduced to basal level.There are important changes in the body during sleep:
- PLASMA VOLUME Plasma volume decreases by about 10% during sleep.
- 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 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.
- RESPIRATORY SYSTEM
Rate and force of respiration are decreased. Respiration becomes irregular and Cheyne-Stokes type of periodic breathing may develop.
- GASTROINTESTINAL TRACT
Salivary secretion decreases during sleep. Gastric secretion is not altered or may be increased slightly. Contraction of empty stomach is more vigorous.
- EXCRETORY SYSTEM Formation of urine decreases and specific gravity of urine increases.
- SWEAT SECRETION Sweat secretion increases during sleep.
- LACRIMAL SECRETION Lacrimal secretion decreases during sleep.
- MUSCLE TONE Tone in all the muscles of body except ocular muscles decreases very much during sleep. It is called sleep paralysis.
- REFLEXES Certain reflexes particularly knee jerk, are abolished. Babinski sign becomes positive during deep sleep. Threshold for most of the reflexes increases. Pupils are constricted. Light reflex is retained. Eyeballs move up and down.
- BRAIN Brain is not inactive during sleep. There is a characteristic cycle of brain wave activity during sleep with irregular intervals of dreams. Electrical activity in the brain varies with stages of sleep.
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5390528/ FUNCTIONS AND MECHANISM OF SLEEP BY NCBI RESOURCE
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4755451/ PHYSIOLOGY OF SLEEP BY NCBI RESOURCE
- TEXBOOK OF PHYSIOLOGY BY GUYTON AND HALL 6TH EDITION
- ESSENTIAL OF PHYSIOLOGY BY SEMBULINGAM