Download Sleep Physiology

Sleep Physiology
Sleep is a 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.
Brain Stem
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.
Diencephalon
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(Electro EncephaloGram),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.
Brain waves
Sleep stages and brain waves
Stage 1.
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.
Stage 2
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.
Stage 3
This is marked by the appearance of Delta waves. The delta waves have high amplitude and show
less than 3.5Hz frequency.
Stage 4
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.