Rapid eye movement sleep

Electroencephalography of a mouse. The MOR dream is characterized by a prominent theta rhythm.

rapid eye movement sleep (REM), paradoxical sleep or unsynchronized sleep, also known as by its acronym in English, REM phase (for Rapid eye movement ), is one of the two stages of sleep. It is a unique phase of sleep in mammals, some birds and other animals which have a pineal gland; characterized by random and rapid eye movements, reduced muscle tone throughout the body, and the person's propensity to dream vividly. Their names paradoxical or desynchronized are due to their similarities to wakefulness, including fast, low-voltage desynchronized brain waves.

The chemical and electrical activities that regulate this phase appear to originate in the brain stem and are characterized by an abundance of the neurotransmitter acetylcholine, combined with an almost complete absence of the monoamine neurotransmitters histamine, serotonin, and norepinephrine. Cortical and thalamic neurons of the awake brain or in paradoxical sleep are more depolarized than in deep sleep.

Before and during REM sleep, PGO (ponto-geniculo-occipital) waves occur, originating from various groups of neurons in the brain stem. They are bursts of electrical activity that can occur as individual potentials or in groups. These waves they reach their greatest amplitude in the lateral geniculate nucleus, the primary visual cortex, and the visual association cortex. Brain energy, measured in terms of glucose and oxygen metabolism, equals or exceeds that used in wakefulness. The one used in NREM sleep is between 11 and 40% lower.

Relation to memory

The hypothesis that sleep participates in the consolidation of recent memory has been investigated using four paradigms:

1. Effects of sleep deprivation on consolidation memory.
2. Learning effects on post-training sleep.
3. Effects of stimulation during sleep over sleep patterns and memory.
4. Reexpression of neural specific behavior patterns during post-training sleep.

These studies convincingly support the idea that sleep is profoundly involved in memory functions in humans and animals. However, the available data are still too sparse to unequivocally confirm or reject the recently put forward hypothesis that non-declarative and declarative memory consolidation, respectively, depend on REM and NREM sleep processes.

Characteristics of the sleep phase

The lightest sleep occurs at this stage; individuals awakened during REM sleep feel alert and rested. Penile or clitoral erections are common during REM sleep, regardless of dream content; heart rate and respiratory rate are irregular, and again similar to the rest of the day, and body temperature is not well regulated and approaches room temperature. REM sleep can occur in mammals and also in birds.

REM sleep is so physiologically peculiar that the rest of the other stages of sleep are collectively known as non-REM sleep or slow wave sleep (SOL), the latter due to the readings in the electroencephalogram.

Polysomnographic record of MOR sleep. Quick eye movements are reflected by the red line.

During a night's sleep, a person typically has four or five periods of REM sleep, very short at the beginning of the night and longer towards the end. It is common to wake up for a very short time at the end of a REM phase (a few seconds). The total REM sleep time per night is between 90 and 120 minutes composed of intervals of seconds in adults, about 8 hours in newborns and up to 15 hours in fetuses.

Physiologically, certain neurons in the brainstem, known as REM sleep cells, are particularly active during this phase and are probably responsible for its creation. During REM sleep, these neurons trigger the release of the MAO enzyme, thus completely inhibiting the release of (catalyzing the oxidation of) certain monoamine neurotransmitters (norepinephrine, serotonin, and histamine). For this reason, motor neurons are not stimulated by brain activity and the muscles of the body do not move.

REM sleep is also seen in other mammals. It appears that the amount of REM sleep per night for each species is highly correlated with the developmental status of newborns. The platypus, whose newborns are completely dependent and undeveloped, have 8 hours of REM sleep per night. In dolphins, whose newborns are fully functional, there is virtually no REM sleep ^{[citation needed]}.

Theoretical proposals of this phase in sleep

At the cellular level:

• Neuronal activity observed during the vigil seems to be reactivated during sleep.
• There is a reactivation of the activity in the hippocampus, neocortex and thalamus. (Memory Formation).

At the behavioral level:

• Exposure to learning tasks increases sleep, especially the paradoxical sleep phase (REM or MOR).
• The deprivation of SOL (slow wave dream) or NREM impairs the consolidation of memory.

According to a third theory, newborn REM sleep provides the neural stimulation necessary for neural connections to mature; hence, animals that are born mature do not need it much. This theory is supported by the fact that the amount of REM sleep decreases with age.