Most of the higher animals sleep. It is a crucial physiological process, evolutionary preserved with minor changes among different animal species. So far, we have been unable to discover the full meaning of sleep. Leading experts agree only to the point that “current science knows only one reason why humans must sleep – because we are tired”.
Sleep is defined as a state of unconsciousness from which a person can be aroused by an adequate sensory stimulus. It is also characterized by a volitional motor inactivity and by a state of relaxation.
Sleep is a cyclic process. It is repeatedly created every night, on a basis of circadian rhythm (see in the text). Even the process of sleeping has a cyclic structure, while there are two qualitatively different types, which change periodically:
1) NREM (non-rapid eye movement) or slow-wave sleep
2) REM (rapid eye movement) sleep (paradoxical/desynchronized sleep)
A person goes through stages of these two types of sleep that alternate with each other a couple of times during each night. Most sleep during each night is of the slow-wave form, therefore the NREM sleep. Approximately after every 90 minutes, an episode of REM sleep, which lasts for 5-30 minutes, appears. REM form occupies circa 25 per cent of the total sleep time.
This deep form of sleep brings us rest and relief. During this repeated phase, the vascular tone, blood pressure (up to 30 per cent), respiratory rate and basal metabolic rate decrease. It is sometimes known as “dreamless sleep”, even though nowadays it is proved that in both types of sleep dreams occur. However, dreams of NREM sleep are likely not to be remembered, because during slow-wave sleep the function of memory consolidation is altered.
Slow-wave sleep may be divided into four stadiums, based on the inner characteristics of each of them (they differ in the depth between each other). Light 1. stadium starts straight after falling asleep; the deepest 4. stadium may appear only in the first one third of the night. The difference may also be seen on EEG (electroencephalography) recordings:
1. stadium: alpha activity followed with sporadic theta waves
2. stadium: theta activity with sleep spindles and K-complexes
3. stadium: sleep spindles prevail with sporadic slow-waves
4. stadium: slow delta activity
After reaching the fourth stadium, the person experiences the stadiums backwards, going through stadium 3, 2 and finally 1 again. Then, for a while REM form of sleep occur.
Rapid eye movement form is called based on its very fast involuntary movements of the eye bulbs. It appears in the episodes after every 90 minutes of NREM sleep. The intervals of NREM episodes are becoming shorter, between the episodes of REM phases, as we sleep longer.
The duration of REM phase depends on the condition of the sleeping person. If the person is extremely tired, REM sleep may be absent completely. During the night, with the increasing state of rest, the duration of REM phase is longer.
There are some very important characteristics of REM sleep:
1) Active form of sleep
This form is often associated with live dreams and the involuntary movements of the skeletal striated muscles. They are often contracted by the peripheral muscles, in the form of clonus.
2) Difficult waking-up part
In the night, it is very difficult to wake up a person during the REM phase. The person usually reacts only on excessively strong or very specific sensory stimuli. Among the specific stimuli belong, for example, hearing our own name or hearing a baby crying. However, people usually awake during an episode of REM sleep in the morning.
3) Depressed muscle tone – it is even more noticeable than in NREM sleep.
4) Irregularity of heart rate and respiratory rate – these features are associated with the dream state.
5) Increased activity of brain
The brain shows higher metabolic rate (increase up to 20 per cents) and EEG recordings are similar to a conscious brain during wakefulness. Therefore, the REM phase is also called the paradoxical sleep.
We still do not know why there are phases of REM sleep during the slow-wave sleep. The agonists of acetylcholine increase the frequency of REM sleep. In the brain stem, there are large neurons (a part of the cholinergic system) projecting into many parts of the brain. Their regular discharge would then cause an excessive activation of innervated areas and increase the rate of metabolism and the activity of the brain. It would also be probably ineffective in the formation of consciousness.
According to a couple of studies, locus coeruleus and noradrenergic system are mentioned, as basic structures required for the REM variant of sleep. Their role in theory is much more difficult to explain compared to the cholinergic system.
EEG recording depicts REM sleep as a desynchronized curve with low amplitude.
Passive theory of sleep
This older theory is no longer accepted by most of the specialists. Sleep induction was explained in the way that the excitatory neurons of the ascending reticular activating system (ARAS), the part of the brain responsible for keeping consciousness, are being exhausted during the day and in the end they stop being active at all. The organism then falls asleep.
Active theory of sleep
This theory was established based on the experiments which proved that if the inhibitory projections leading to the reticular activating system are interrupted, the cerebral cortex loses its ability to sleep. Therefore, sleep is not induced by a simple exhaustion of ARAS, but by its active inhibition caused by projections from different parts of the brain.
Stimulation or destruction of some structures of the central nervous system proved that their function is strongly associated with sleep.
This unpaired nucleus is localized in the central line of the brain stem. Unpaired fibers from the nucleus form association fibers, which connect nucleus raphe with surrounding structures of the brain stem; and also the projection fibers that terminate in the thalamus, hypothalamus, in the majority of the structures from the limbic system, and in the large portion of the cortex. It also has descending projections to posterior spinal horns, in which the sensory inputs are suppressed.
This nucleus is a part of the serotonergic system (respectively most of its neurons produce serotonin). If the experimental animal is injected with the antagonist of serotonin, it will have a couple of days of insomnia.
Natural sleep occurs after a stimulation of nucleus raphe; variants of REM and NREM phases alternate.
Nucleus tractus solitarius
It is another nucleus from the region of the brain stem, whose stimulation induces sleep. In the nucleus tractus solitarius terminate most of the sensory input conducted by nervus vagus or nervus glossopharyngeus.
Even in this case, the stimulation of some zones from the hypothalamus results in sleep, especially the rostral and suprachiasmatic parts.
Common and important characteristic of the centers of sleep is their inhibitory influence on ARAS. It is spontaneously discharged during the day; it excites cortex and other structures, which project back and excite ARAS. Thanks to this mechanism, the neuronal oscillator is created with a positive reverse feedback. Conscious, awake state of mind is therefore kept until the activity of the oscillator is interrupted by an external influence – specifically by the increase of the inhibitory activity in the centers of sleep. It is believed that the exhaustion of the synapses in the neuronal oscillator ARAS – cortex is partly involved in the sleep induce as well.
In the beginning of this chapter we mentioned that the function of sleep is still unknown. At the same time, we are aware of the fact that it is essential for the organism. From the data, we unfortunately have out of experiments conducted on prisoners; we know that a person after 14 days of sleep deprivation dies. The reason why is uncertain. Even after a couple of hours of sleep deprivation, a rapid decrease in mental and physical abilities of human beings occurs.
Lack of sleep usually follow with a so-called rebound phenomenon, while during an upcoming sleep, longer than usual, dominates the phase which was suppressed before. Usually it is the NREM type, because it occupies up to 75 per cent of the sleep time. In laboratory, we can also suppress the REM phase; rebound phenomenon then mainly compensates this phase.
Due to the fact that a long-term restriction of sleep results in a higher irritability and psychotic signs (hallucinations, delusions), we can state that sleep, thanks to a so-far unknown mechanism, keeps the normal activity of the systems of the CNS. Other postulates mention that sleep participates in maturation of neurons, learning facilitation, memory consolidation and energy conservation. For every out of these postulates exist numerous studies which support them and many which disproves with them.
For example, the postulate about the energy conservation is no longer accepted. The energy consumption during this phase is almost the same as the basal metabolism in a normal conscious state. Moreover, there are reliable proves about some species of bears which regularly wake up from the state of hibernation during the winter time to fall asleep. Sleep does not occur during hibernation.
Life on Earth had been subordinated to changes of the day and night, to the seasons of the year, etc. during the evolution. Many of the physiological functions had then adapted to these biological rhythms.
Biological rhythms can be divided either based on the autonomy of the pacemaker or based on the duration of the period. Based on the autonomy of the pacemaker, we distinguish between rhythms endogenous or exogenous:
These rhythms are usually given only by a pacemaker; its activity does not interfere with a synchronizer (such variable of the external surrounding, which sets the phase of the rhythm in every single moment).
These rhythms, on the contrary, depend on the synchronizer, without which they lose regularity and the period stops being constant.
Based on the duration of the period, we distinguish between the rhythms circannual, infradian, circadian and ultradian.
Circannual rhythms last for one year.
Infradian rhythms have a period longer than 24 hours. The prefix infra- is used in a connection with frequency.
Circadian rhythms have a period of approximately 24 hours (in Latin circa – approximately, dies – a day).
Ultradian rhythms have a period shorter than 24 hours. The prefix ultra – (more-) is used with regards to frequency, not to a period.
In the end of this subchapter we will focus on the circadian rhythm, because it is the most important one for the human organism.
Pacemaker of this rhythm is the change in the concentration of melatonin, a hormone which is being produced in the epiphysis. Synchronizer of this rhythm is sunlight, absorbed in the retina. The signal is then transported thanks to the retinohypothalamic tract to the suprachiasmatic nucleus, where it is then switched to epiphysis.
The journey from the suprachiasmatic nucleus to the epiphysis is very complex. The information travels through a spinal cord to the superior cervical ganglion and its perivascular sympathetic fibers finally innervate the epiphysis. The release of noradrenaline then stimulates its neurons to convert serotonin to melatonin and to dismiss the hormone. The formation of melatonin is closely described in the Subchapter 12/4.During the night, the activity in sympathetic fibers is increased to reach the maximum around a subjective midnight. The light, on the contrary, lowers the number of excitements, which get to epiphysis, and the synthesis of melatonin decreases.
Epiphysis is able to have a pacemaker activity independent of external synchronizer. If an individual is monitored during an experiment in a completely dark environment, the period of the rhythm will be set to approximately 23 hours.
It is supposed that due to a change in the concentration of melatonin, time is set in every moment. It is an example of the inner clock.
Melatonin itself has very interesting characteristics. It is an important scavenger of free radicals, which acts as a terminate antioxidant. Compared to the vitamins C and E, it therefore does not undergo the redox cycling. Its maximal concentration is lowered in people who stay awake during nights. This finding correlates with the higher prevalence of tumorous diseases in these individuals.
Desynchronization of Circadian Rhythms
During the flight across time zones the desynchronization of the circadian rhythm, a so-called fly lag disease, occurs. Affected people have problems with falling asleep. This condition may be improved by providing the person with a single-dose of melatonin before the required time of falling asleep. It stimulates the natural increase in the concentration during the night, because melatonin reabsorbs slowly out of GIT. In general, our organism deals worse with a flight to the east; where the adaptation takes longer time.
Subchapter Author: Patrik Maďa