04. Sleep

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Sleep [ 91 , 119 ] is a loss of normal consciousness that differs from coma by its reversibility. Indeed, even in the deepest stages of sleep, a sufficiently strong stimulus can be perceived by the brain and awaken the sleeper [ 1 ]. The sleep-wake cycle is one of the fundamental cycles in humans and almost all animals: the circadian rhythm [ 38 , 119 ].

1. Sleep time:

We sleep at least a third of our lives [ 41 ], it suffices to prove the importance of sleep.

In a normal adult, the average sleep time is 7 to 8 hours of sleep per day [ 42 ]. Long sleepers may need 10 hours of sleep while short sleepers may just 5 hours [ 168 ].

There is no real optimal sleep duration, the only criterion for a good amount of sleep to feel like the day after.

Sleep duration varies with age [ 145 ]. Basically, from birth to death, it only decrease.

2. Stages of sleep:

Far from uniform, our sleep fluctuates between different stages (or phases) [ 38 , 41 , 57 ] occurs in a characteristic order during the night.

The electroencephalogram (EEG) [ 41 ] is the most widely used test to study sleep. The AHI (sleep EEG) [ 5 , 136 ] to record the activity of cortical neurons in using electrodes arranged in specific areas of the scalp.

The EEG has identified two main phases of sleep [ 80 ]: The slow wave sleep and REM sleep [ 166 ].

Waves EEG allow according to their frequency and amplitude to distinguish four phases of REM sleep [ 1 , 72 ]: Stage I corresponds to sleep or drowsiness, light sleep stage II and stage III and IV correspond to deep sleep.

During REM sleep, the plot of the EEG [ 4 ] resembles that of awakening with its fast pace and low amplitude. It is for this reason that the neurobiologist Michel Jouvet appointed REM in 1959 [ 119 ]. Anglo-Saxon refer to it as the name of REM (Rapid Eye Movement in English [ 32 ]) because this type of sleep is characterized by many rapid eye movements under closed eyelids.

The different stages of sleep are conducted as follows: 1-2-3-4-3-2-1-SP-1-2-3-4-3-2-1-SP-1-2 ... etc.. Each run deep sleep is followed by a rise leading to a period of REM sleep [ 5 ].

Although similar duration, cycles change during the night. In the first third, the REM sleep dominates [ 39 ]. In fact, the first two cycles have essentially deep sleep [ 57 ], this is what explains the great virtues of physical recovery associated with the first hours of sleep.

In return, light slow wave sleep and REM sleep are proportionally higher end of the night [ 57 ].

3. Sleep and activity:

Sleep is far from simple shelving of our physical and mental activity. There is a second condition as varied and complex as the waking state [ 57 ] and is associated with significant physiological changes (temperature, hormone secretion, heart and respiratory rate, etc.). [ 41 ].

3.1. During REM sleep:

Muscles are more relaxed, and the few movements only serve to adjust the position of the body. The general metabolism of the body decreases as temperature, energy consumption, heart rate, respiration, kidney function, all this slows according to the predominance of the parasympathetic system during this phase of sleep.

The slow rhythms of the EEG during REM sleep indicates that the brain also seems to rest.

3.2. During REM sleep:

The oxygen consumption by the brain (which reflects its energy consumption) is very high, even higher than that of an awake brain that thinks about a complex cognitive problem.

There is almost complete loss of muscle tone that makes us literally paralyzed, which prevents the body to update dreams!

However, respiratory and cardiac muscles provide vital services. Eye muscles remain active and produce the famous rapid eye movements, and heart and respiratory rate increases erratically under the influence of the sympathetic nervous system.

4. Role of sleep:

There are very few certainties about the role of sleep [ 1 ]. Slow wave sleep and REM sleep have different roles completions.

While during REM sleep, the body goes to rest, why deep sleep is also called sleep. The role of REM sleep is much more controversial, it seems that serves to consolidate the processes of memory and learning [ 163 ], it also plays a role in emotions and personality traits, finally it is the support privileged dreams.

The role of dreams is also controversial [ 5 ] Freud describes dreams (royal road to the unconscious) [ 76 ]. Their scenario would be made ​​from impressions experienced during the day and old memories that are processed or disguised in order to escape the control of consciousness.

Other scientists be given an arbitrary character which derives from the wrong interpretation by the brain signals from outside view of consciousness disturbed sleep.

Dreams do not occur only during REM sleep, but it is in this phase that has the clearest scenes and images and the most consistent events [ 57 ], deep sleep is the breeding ground of nightmares and night terrors [ 166 ].

5. Mechanisms of sleep:

The mechanisms of sleep and wakefulness are very complex [ 5 , 95 ], they involve several anatomical structures and several chemical molecules.

5.1. Anatomical structures involved:

Very schematically, the components of modulatory systems of sleep-wake cycle can be grouped into two major pathways that have both origin as part of the reticular nucleus of the medulla oblongata.

  • The ventral pathway [ 172 ] which projects towards the posterior hypothalamus and the nucleus of Meynert basal forebrain [ 119 , 172 ] (acetylcholine neurons). This is the reticulo-hypothalamic-cortical pathway. Stimulation of the posterior hypothalamus produces a state of arousal comparable to that obtained by stimulation of the reticular formation of the brainstem.
  • The dorsal pathway [ 172 ] active nuclei mésopontins cholinergic [ 119 ], the midbrain reticular formation (neurons aspartate / glutamate) and the thalamus. This is the reticulo-thalamic-cortical pathway.

A key structure of the sleep-wake cycle is the suprachiasmatic nucleus [ 57 ], it belongs to the hypothalamus where it is located directly above the optic chiasm [ 119 ]. The activity of neurons in the nucleus increases and decreases over a period of about 24 hours, a nucleus seems key element in the regulation of circadian rhythm.

The suprachiasmatic nucleus receives signals from the optic nerves to calibrate the biological clock [ 1 , 39 , 119 ] from the day-night cycles. It is projected into a set of regions of the brain: hypothalamus, brainstem and midbrain. It also influences the secretion of melatonin (key circadian hormone) by the pineal gland [ 57 ].

The locus coeruleus [ 38 ] (a nucleus located at the pons) also plays a major role in the sleep-wake cycle through its connections with the raphe nuclei [ 38 ] (involved in sleep and projecting to the suprachiasmatic nucleus), the paraventricular nuclei of the hypothalamus and the brain neocortex. These projections are adrenergic in nature which makes the locus coeruleus a true nucleus stress [ 80 ], motivation and arousal. The entire destruction of this nucleus suppresses REM sleep and dream.

5.2. Biology of sleep:

Two processes must overlap properly in the body so that we can fall asleep:

  • The circadian rhythm set by our biological clock, which orchestrates the cyclic secretion of several hormones, including melatonin [ 1 ], involved in sleep.
  • The accumulation of substances hypnotics during the day, substances that induce sleepiness are disappearing with sleep, such as serotonin, which is secreted by the anterior hypothalamus and inhibits the posterior hypothalamus [ 136 ].

One of the most studied factors hypnogenic is adenosine [ 136 ], a small molecule derived from the degradation of ATP (main energy substrate cells). Adenosine also acts as a neuromodulator in many synapses in the brain. Natural antagonists of adenosine receptors as caffeine from coffee or tea theophylline are well known for their stimulant substances.

The activity of the posterior hypothalamus decreases naturally during sleep, less then releasing histamine [ 41 ]. Antihistamines that you take against allergic manifestations also cause some drowsiness by decreasing the activity of histamine [ 74 ].