Rapid eye movement (REM) sleep is marked by movement of the eyes and low muscle tone in the body, in addition to more rapid brain waves, similar to that of an awake individual.
During this stage of sleep, an individual has dreams and the brain produces waves similar to that of a person who is awake, explaining why it is sometimes called paradoxical sleep. This is in contrast to non-rapid eye movement sleep, which encompasses the other stages of sleep and is associated with distinct differences brain waves, which are slower and more rhythmic.
When an individual is in REM sleep, the voluntary muscles in the body become paralyzed, and the nervous activity of the brain intensifies in respect to the other stages of sleep, similarly to the brain activity while awake.
It is believed the REM sleep is triggered by neurons in the brain stem called REM sleep-on cells. At this point, monoamine neurotransmitters cease to be readily available, resulting in lack of stimulation of the motor neurons and, subsequently, paralysis of the voluntary muscles.
The neurotransmitter acetylcholine is more active, which is likely to be linked as a causative factor for faster brain waves. Conversely, other neurotransmitters such as norepinephrine, serotonin and histamine are not available.
The neurons in the cortical and thalamic regions of the brain are more depolarized and are readily activated when an individual is awake or in REM sleep, which is suggestive that the brain activity during REM sleep has an important role in the healthy function of the brain.
During REM sleep, regular homeostatic processes are altered, resulting in changes in heart rate, breathing rate and blood pressure. In particular, the brains response to signs of hypoxia are reduced, and there is less control over the rate of breathing and electrical signals in the brain do not impact the action of the lungs as in the other stages of sleep and during waking. The cardiovascular changes appear to be linked to the rapid eye movement and changes in breathing.
Additionally, the body temperature is less well regulated during REM sleep and, as a result, individuals are more sensitive to environmental temperatures during this time. This is due to the inactivity and lack of firing of thermoregulation neurons that usually fire in response to extreme temperatures, and the paralysis of voluntary muscles cannot provide additional heat.
REM atonia refers to the broad paralysis of the voluntary muscles during REM sleep, which occurs as a result of motor neuron inhibition. The neurons become hyperpolarized during this stage of sleep, increasing the membrane potential and the difficulty for them to become activated.
REM sleep is thought to play a role in several functions of the brain, including the consolidation of memory and the development of the brain and central nervous system.
During this stage of sleep, people are most likely to have vivid dreams, which is thought to be associated with the process of stabilizing spatial and procedural stimuli into memories. Additionally, it has been suggested that it may help to reduce abnormal modes of neural interaction in the cerebral cortex.
It has also been noted that infants and young children spend a greater proportion of their sleeping time in REM sleep, which suggests that it is important for the development and growth of the brain. REM sleep may be responsible for stimulating the neural connection required for brain maturation.