In this essay we will discuss about the types and theories of sleep.
Type of Sleep:
Sleep is an altered state of consciousness during which some of body functions are depressed and some others activated.
i. Sleep and coma:
A person who is in sleep can be aroused by tapping and a person who is in a state of coma cannot be aroused even by a painful stimulus.
Sleep can be a slow wave sleep (NREM) or a rapid eye movement (REM) sleep. Usually these two types of sleep are seen one following the other. And such 5 to 7 cycles alternate during entire sleeping hours.
Based on EEG recordings during sleep, slow wave sleep is divided into four stages, namely stage 1 to stage 4. Stages 1 and 2 will be light sleep; stages 3 and 4 will be deep sleep stages (Fig. 9.57).
This sequence is followed when a person continues to sleep, but EEG recording suddenly changes to high frequency, low amplitude waves showing brain is alert during REM sleep even though person is in a state of deep sleep— therefore, this is known as paradoxical sleep. This is usually associated with rapid eye movements and dreams. Hence it is often called REM sleep.
ii. Age and sleep patterns:
Duration of sleep declines with age. A newborn spends most time in sleep; at least 50% of this duration is of REM type of sleep.
In puberty, duration of sleep is reduced. Approximately 25% of this duration is of REM type. After the age of 60, total duration of sleep is markedly reduced. The duration of REM sleep is almost negligible. At this age, most of time is spent in stages 1 and 2 (light sleep). Barbiturates which are commonly used as sleeping pills suppress stages 3 and 4 of sleep. It also suppresses REM sleep.
Progressive changes in EEG:
Slow wave sleep—stage 1, low voltage high frequency wave resembling that of alert state.
Stage 3—appearance of sleep spindles and low frequency, high voltage waves.
Stage 4—very high voltage waves with a marked decrease in frequency refer to as delta waves (Fig. 9.58 and Table 9.6).
Importance of REM sleep:
Suppressed emotions will be vent out during this sleep and hence normalize the balanced activity of neurons. If this cannot occur, person becomes highly irritable and complains of insomnia and this may lead to pure psychosis.
A newborn needs no teaching as to how to suckle. It has learnt the same during its REM sleep in uterus.
Theories of Sleep:
Passive theory:
Experimental evidences show that when all sensory inputs to ARAS are cut off, ARAS activity markedly decreases resulting in decreased activity of cortical neurons and hence resulting in sleep.
Present view:
Process which results in sleep is an active one.
Stimulation of:
a. Raphe nucleus
b. Nucleus of tractus solitarius
c. Preoptic and suprachiasmatic nuclei in hypothalamus result in sleep.
Impulses from raphe nucleus go up to thalamus, reticular formation and all parts of cerebral cortex, hypothalamus and limbic system.
Transmitter substance involved is 5 HT (drugs that block synthesis of 5 HT, the animal cannot sleep). But there are some reservations about involvement of 5 HT, because when a sample of CSF is obtained during sleep, there was no increase of 5 HT in CSF. Further research has shown that muramyl dipeptide may be a candidate, interleukin associated with bacterial infections may also be substance.
When there is lesion in raphe nucleus, suprachiasmatic nucleus, and anterior hypothalamus, inhibition on ARAS are removed and animal will be in arousal state throughout.
Sleep inducing areas of brain include:
a. Raphe nucleus
b. Nucleus of tractus solitarius
c. Suprachiasmatic nucleus and preoptic nucleus
And also stimulation of cutaneous nerve fibers (inducing sleep in babies)
Neurotransmitters involved:
1. Raphe nucleus—5 HT, slow wave sleep
2. Preoptic nucleus—5 HT, REM sleep
3. Sleep producing substances are—muramyl dipeptide (MDP), interleukin I
REM sleep is due to release of ACh from neurons of upper ARAS. This activates many parts of brain.
Physiological importance of sleep:
Prolonged suppression of sleep will result in disruption of process of thinking, followed by behavioral alterations. Person becomes more irritable and at time may lead to psychosis. Sleep restores natural balanced activity of CNS neurons.