Why We Sleep
Table of content:
| 1. | Definition |
| 2. | Types |
| 3. | NREM Sleep Features of NREM sleep: Stages |
| 4. | REM Sleep Features of REM sleep: |
| 5. | Sleep cycle |
| 6. | Sleep-waking cycle Mechanism |
| 7. | neurotransmitters andNeuronal centers |
| 8. | EEG pattern |
| 9. | Factors promoting sleep |
Definition:
Sleep is a natural physiological reversible condition in which unconsciousness occurs from which sensory stimuli can arouse a person. Sleep occurs regularly, and it is regulated homeostatically. During sleep body rests, recovers, and restores itself. Sleep is a reversible state of altered consciousness.
Three main features of sleep are:
1. Consciousness is partially or suspended and is reversible.
2. Excitability is wholly or partially reduced, and the stimuli threshold is increased.
3 All most physiological functions are reduced.
Sleep is different from a coma. Coma is not a physiological condition in which unconsciousness occurs, and a person cannot be aroused.
Types of sleep:
Sleep is divided into two entirely different but closely related significant groups. They have different qualities associated with each other and alternate with each other.
1. Slow-wave sleep or Non-Rapid Eye moment sleep(NREM).
2. Rapid Eye Movement sleep (REM)-=Paradoxical or dreaming sleep.
In Non-Rapid Eye Movement sleep, or slow waves sleep. ECG waves are strong and of low frequency. Muscle tone progressively decreases. A person does not report dreaming.
In Rapid Eye Movement sleep, a person sleeps, but his eyes move rapidly. There are marked brain activities, so it is known as paradoxical sleep. Rapid Eye Movement sleep occurs in episodes. The episodes usually require about every 90 minutes.REM sleep is not very restful and is associated with vivid dreaming.
We will learn and discuss NREM and REM sleep in detail later on.
Sleep cycles
In an average person with six to eight hours of sleep, there are many sleep cycles of NREM and REM sleep. One sleep cycle lasts about 90 minutes, of which 25% is occupied by Rapid Eye Movement sleep. The duration of Rapid Eye Movement sleep increases progressively with each successive sleep cycle.
The depth of sleep progressively decreases with each sleep cycle.
REM sleep reduces in old age.
Electroencephalography in different stages of sleep:
Stages of Non-Rapid Eye Movement, according to Rechtschaffer and Kales (1974) Non-Rapid Eye Movement was divided into four phases. Likewise, ‘The American Academy of sleep medicine’ divides NREM sleep into three stages.
From S2, it moves to REM sleep.
S1 -> S2 -> S3-> S2-> REM
Stage 1. is light sleep
Stages 2 and 3 are between light and deep sleep.
Stage 4 is deep sleep.
EEG pattern:
In alert wakefulness, EEG is characterized by high-frequency beta waves. Each successive stage has an ECG pattern with a slower frequency and higher voltage than the preceding one.
In quiet wakefulness with eyes closed, alpha waves are present in EEG.
When a person opens their eyes from quiet wakefulness, Theta waves appear.
The first stage of NREM sleep is light sleep.
In this stage, EEG wave voltage is reduced and interrupted by sleep spindles. Sleep spindles are short spindle-shaped bursts of alpha waves. This is a sleep spindle and occurs periodically.
In 2,3,4 stages of slow wave sleep, the frequency of EEG waves becomes progressively slower and reaches a frequency of 1 or 2 waves per second.
In the 4th stage of NREM sleep, Delta waves appear.
During REM sleep, EEG waves resemble that of an active, awake person. This is because the waves are irregular and of high frequency. This is desynchronized EEG. Therefore, REM sleep is also known as desynchronized sleep. In this sleep, the brain is very active.
When a person is very sleepy, REM sleep is short, and REM frequency increases as the person becomes more rested.
Features of REM sleep:
REM sleep is the active form of sleep in which the brain is very busy.
Although muscle tone is very depressed, active body movements are present.
It isn’t easy to arouse by sensory stimuli.
People usually awaken spontaneously in the morning during an episode of Rapid Eye Movement sleep.
Rapid movements of the eyes occur, which is known as ‘saccadic eye movement.’ Saccadic eye movements are small jerky, rapidly moving eyes from one fixation point to another.
Muscle tone is decreased. During sleep, the tongue in a supine position partially obstructs the air passage, causing snoring.
In children, teeth grinding ( bruxism) may occur.
Rapid Eye Movement is associated with enormous phasic potentials in groups of 3 to 5 that originate in the pons and pass rapidly to the lateral geniculate body and from there to the cerebral cortex. They are ‘Ponto- Geniculo-occipital spikes.’ This causes hypotonia during sleep.
REM sleep is associated with increased blood pressure, irregular heart, and respiratory rates.
Dreaming is closely related to REM sleep.
Rapid Eye Movement sleep is essential for mental well-being.
Rapid Eye moment is essential for long-term chemical and structural changes that the brain must undergo to make learning and memory possible.
NREM sleep:
Slow wave sleep, or NREM, is dreamless sleep.
Sleep is very restful and is associated with decreased physiological functions of the body, causing a reduction in blood pressure, decrease in heart rate, respiratory rate, and ‘basic metabolic rate.’
Tidal volume and pulmonary ventilation decrease.
Urine is more concentrated in the morning.
Muscles are completely relaxed, and the tone is minimal.
Superficial and light reflexes are not affected, but deep reflexes are diminished.
Eyelids come closer.
Pupils constrict.
Eyeballs roll up and out due to flaccidity of the external ocular muscles.
During slow-wave sleep, the pulsatile release of growth hormone and gonadotrophins from the pituitary gland occur. That indicates that slow-wave sleep is essential for the body’s rest and metabolic restoration.
The excited area of the upper brain stem, the reticular activated system, produces wakefulness. When the excited area of the brain gets exhausted, it makes sleep.
How sleep is produced:
The following mechanisms make NREM sleep:
1. Descending pathways arising from Broca’s preoptic area and diagonal band inhibit the Reticular activating system and cause sleep.
The preoptic area and the diagonal band of Broca, known as the Basal forebrain sleep zone, prevent desynchronizing activity.
2. Sleep promoting mechanism states that the stimulation of sleep-promoting areas of the brain causes sleep. The rhythmic discharge of impulses from the thalamus is responsible for synchronizing activity.
Sleep promoting mechanism is controlled by :
Diencephalic sleep zone, present in the posterior hypothalamus, intralaminar and anterior thalamic nuclei and
Medullary synchronizing zone, present in the reticular formation of the medulla oblongata.
The following mechanism produces REM sleep:
Pons, amygdala, and anterior cingulate gyrus are very active during REM sleep, which indicates that they are responsible for the production and maintenance of REM sleep.
Neurons present in the pontine reticular formation and in the locus ceruleus secrete Nor-epinephrine and
Pontine cholinergic neurons secrete acetylcholine, essential for shifting NREM sleep to REM sleep.
Ponto-Geniculo-Occipital (PGO)spikes are large phasic waves that originate from the pons and swiftly in the Lateral geniculate body and thence to the occipital cortex. This stimulates the reticular inhibiting area in the medulla oblongata and produces hypotonia.
In short
RAS leads to awakening; its inhibition promotes sleep.
Serotonin secreted from raphe nuclei promotes sleep.
Sleep-promoting peptides accumulate during waking; when their concentration reaches the threshold level, sleep is produced.
Transaction of the brainstem as the level of the mid phones creates a brain cortex that never sleeps. This shows that there is a Centre located below the midpoint level, which is responsible for producing sleep by inhibiting some parts of the brain.
Neuronal centers
1. Stimulation of some areas of the brain produces sleep-like natural sleep.
Raphe nuclei: They are present in the midline of the lower half of the pons and the medulla. The secret serotonin. Neuronal fibers from this reticular formation spread in the
Brain stem reticular formation
Hypothalamus
Neocortex of the cerebrum
Most parts of the limbic system
In the spinal cord, posterior horn cells
2. Some areas in the ‘nucleus of the Tractus are solitarious when stimulated, causing sleep.
3. Neurons in the rostral part of the hypothalamus, mainly in the suprachiasmatic areas, and some diffuse nuclear of the thalamus.
Mode of action, the excitatory reticular nuclei of the mesencephalon and upper pons are released from the inhibition leading to intense wakefulness. Other neurotransmitters are :
Muramyl peptide, nano peptide, and sleep factors isolated from the blood of sleep-deprived animals can produce sleep.
Why is slow-wave sleep interrupted periodically by Rapid movement sleep?
Large acetylcholine secretory neurons in the upper brain stem reticular formation activate any portions of the brain, causing Rapid Eye movement and sleep.
Sleep-waking cycle
Periods of awake and sleep alternate once a day in an average person. This is a Circadian rhythm consisting of 8 hours sleep and 16 hours awake. This circadian rhythm is controlled by the ‘biological -clock’ of the hypothalamic suprachiasmatic nucleus.
Sleep waking cycle consists of
1. Arousal system and
2. Sleep-producing system.
The arousal system is activated by:
Sensory receptors stimulation, stimulation of midline reticular formation of the brain stem, raphe nuclei, hypothalamus, and locus ceruleus.
Sleep producing system is activated by
Removal of sensory stimuli, which in turn reduces RAS (reticular activating system) activity.
Stimulation of anterior and dorsal hypothalamic areas.
The primary neurotransmitters involved in sleep-waking cycles are:
Epinephrine, acetylcholine, and serotonin.
Other chemical substances are:
Sleep promoting factor -factor-S, delta-sleep inducing peptide (DSIP), Orexin, muramyl peptide.
Mechanism
The mechanism of this sleep-waking system needs to be clearly understood. The one widely accepted theory suggests that when the sleep centers are not activated, the mesencephalic and upper pontine reticular activating nuclei are released from their inhibition, allowing the reticular activating nucleus to become active. This spontaneous activity, in turn, excites the cerebral cortex and the peripheral nervous system. They, in turn, send numerous Positive feedback signals back to the same reticular activating nuclei to activate them further. Therefore once wakefulness starts, it has a natural tendency to sustain itself due to the positive feedback activity.
The brain remains active for many hours. However, neurons in the activating system become fatigued after a few hours. Consequently, the positive feedback cycle between the mesencephalic and upper pontine reticular activating nuclei and the cerebral cortex fades, and the sleep-promoting effects of the sleep centers take over, causing a rapid conversion of wakefulness to sleep. They are also responsible for the quick change from sleep to wakefulness.
Hypothalamic neurons produce Orexin that excites many other areas of the brain. Orexin is also known as hypocretin. Orexin combines with Orexin receptors present in the brain and stimulates them. Orexin-producing neurons are active during wakefulness and almost silent during sleep with non-rapid eye movement.
Factors promoting sleep:
Reduced sensory inputs, tiredness, serotonin-producing activities, and a calm mind promote sleep.
1. Comfortable room and bed.
2. Noise-free, comfortable temperature and environments.
3. Relaxed body and mind.
Anxiety and emotions prevent the onset of sleep by producing epinephrine which activates the reticular activating system.
4. Physical activities promote sleep despite unfavorable surroundings.
