Sleep is the physiological process by which body gets sufficient rest to rejuvenate the cells including the brain cells. It is actually a shift in brain’s activities to slow the basic bodily functions. This shift in the brain’s activity is accompanied by a major shift in the consciousness.
Sleep can be defined as a state of unconsciousness from which a person can be aroused by sensory or other stimuli. There are different stages of sleep during which the electrical activities of the brain vary considerably. During each night, every person passes through two stages of sleep.
1. Slow wave sleep: This is characterised by very strong brain waves with very low frequency. Slow wave sleep occurs during the first hour of sleep and is restful with decreased peripheral vascular tone. Ten to thirty percent decrease in BP, respiratory rate, basal metabolism etc occurs in slow wave sleep. Generally slow wave sleep is considered as” dreamless sleep” but occasionally dreams appears without less bodily activities so that these dreams will not be remembered.
2.REM (Rapid Eye Movement) sleep: During this stage of sleep the eye balls move rapidly, even though the person is in full sleep. REM sleep is referred to as Paradoxical sleep or Desynchronized sleep. During the entire sleep time, bouts of REM sleep occur five to thirty times at an interval of 90 minutes. REM is sleep is considered as” Dream sleep” and the dream can be remembered since it involves bodily activities. The important characters of REM sleep are
a. Active muscle movements and dreaming
b. Sensory stimuli will be absent so that it is difficult to arouse through touch, sound or light stimuli.
c. The spinal muscle control area will be inhibited so that the person shows depressed muscle tone.
d. Heart rate and respiratory rate diminishes or irregular.
e. In addition to rapid eye movements, irregular muscle movements also occur.
f. Bain metabolism increases to 20 percent and brain becomes highly active. Due to the increased activity of brain during the sleep, REM sleep is called paradoxical sleep because it is a paradox.
How do we sleep?
There are theories regarding the mechanism of sleep. According to the passive theory of sleep, the reticular activating system of the upper brain is fatigued due to the activities in the wakeful state. But according to the current view, sleep is induced by the active inhibitory centre located below the mid pondile level of the brain stem.
Sleep inducing factors
Several specific areas of brain are involved in inducing sleep. One important area is the Raphe nuclei in the pons and medulla.
Role of Raphe nucleus
The Raphe nuclei are aggregates of special nerve cells and the nerve fibres from the Raphe nuclei pass into the thalamus, hypothalamus, brain stem and most areas of the limbic system. Nerve fibres from the Raphe nuclei also spread into the cerebrum and to the spinal cord where they inhibit the incoming sensory stimuli including pain. The nerve ending of these fibres secrete Serotonin, the sleep inducing neurotransmitter.
Nucleus of Tractus solitarius
This is the termination in the pons and medulla for visceral sensory signals entering through vagus and glosso pharyngeal nerves. Stimulation of the Nucleus of Tractus solitarius induces sleep.
Several regions of diencephalon such as rostral part of hypothalamus, occasional area in the diffuse nuclei of thalamus are involved in the process of sleep.
Electrical activity during sleep
Several changes in the electrical activities take place during the different stages of sleep as experimented in human volunteers using EEG(ElectroEncephaloGram),EMG(ElectroMyoGram) and EOG ( ElectroOculoGram).These studies revealed that the brain waveform changes dramatically during the different stages of sleep. When a person is fully in the awake stage, his brain produces beta waves characterized by high frequency of 14-30 cycles with low voltage. When the person enters into the rest stage before the sleep, the beta waves will be replaced with alpha waves. The alpha waves have lower frequency of 8-12 Hz but have higher voltage level or amplitude.
Sleep stages and brain waves
This is the initial stage of sleep. It is characterized by the appearance of theta waves of 3.5-7.5 Hz. Stage 1 of sleep is the border line between the wakefulness and sleep.
The second stage of sleep generally occurs after 10 or 15 minutes if the person is in stage 1. Stage 2 is marked by the occurrence of Sleep spindles and K-Complexes. Sleep spindles are short bursts of waves of 12-14Hz that occurs 2 to 5 times per minute. The Sleep spindles show the activity through which the brain’s sensitivity to sensory inputs is inhibited. K-Complexes on the other hand are sudden sharp waveforms one per minute. It consists of a brief high-voltage peak, usually greater than 100 µV, and lasts for longer than 0.5 seconds. K-complexes occur randomly throughout the stage 2 sleep, but may also occur in response to auditory stimuli.
This is marked by the appearance of Delta waves. The delta waves have high amplitude and show less than 3.5Hz frequency.
The delta activity progresses as the person enters into the stage 4 of sleep. In stage 3 around 20 percent delta activities occur while in stage 4, it increases to 50 present. After 90 minutes of delta activity, the person enters into the REM sleep. During REM sleep, the brain’s activity increases rapidly and the brain is almost like in the wakeful state. The delta waves disappear and fast low voltage waves appear. The most important aspect of REM sleep is the fast eye movements. Muscular movements also occur but the person is unaware of the surroundings, since the sensory inputs are blocked. Dreams may also develop during the REM sleep. During the rest period, REM sleep continues and alternate with other stages till morning.