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REM sleep was identified and defined by Nathaniel Kleitman and Eugene Aserinsky in the early 1950s and is a normal stage of sleep characterised by the random movement of the eyes. REM is the phase of sleep where vividly recalled dreams mostly occur. It is classified into two categories, namely tonic and phasic. REM sleep is physiologically different from the other phases of sleep, which are collectively referred to as non-REM sleep (NREM sleep). During REM, the activity of the brain's neurons is quite similar to that during waking hours and for this reason, the REM-sleep stage may be called paradoxical sleep.
The criteria for REM sleep includes rapid eye movement, but also low muscle tone and a rapid, low-voltage Electroencephalography (the recording of electrical activity over the scalp). These features are easily discernible in a polysomnogram (the sleep study typically done for patients with suspected sleep disorders).
REM sleep in adult humans typically occupies 20 to 25% of total sleep so it amounts to between 90 to 120 minutes of a night's sleep. During a normal night of sleep, humans usually experience about four or five periods of REM sleep. They are quite short at the beginning of the night and get longer toward the end.
Many animals and some people tend to wake, or experience a period of very light sleep for a short time immediately after a bout of REM. The relative amount of REM sleep varies considerably with age. A new-born baby spends more than 80% of total sleep time in REM.
In 2004 a fresh theory of hypnosis emerged which suggested that hypnosis is the result of accessing the REM (Rapid Eye Movement) state. In REM we access the imagination that is responsible for our dreams. One of the functions of dreaming is that it allows us to complete our unresolved emotions of the day through the metaphoric imagery and connections of our dream. Its other key function is to update our instinctive templates or behavioural and emotional responses. In other words the REM state is also a learning state. Whenever we act without conscious effort, we are reliant on pattern matching by going back to an earlier learned response or behaviour that was set in the REM state. So when we act instinctively, we are in effect acting on a post hypnotic suggestion. This means that when someone is hypnotised, they are simply activating the same processes that the brain activates during dream sleep, and this is what makes hypnosis so effective.
Even though to us sleep may seem like one long continued state of unconsciousness, it occurs in a recurring cycle of 90 to 110 minutes and is divided into two main categories of Rapid Eye Movement (REM) and Non Rapid Eye Movement (NREM).
The first phase of the sleep cycle we experience is known as non-REM sleep, which occurs in four key stages.
The first of the phase is often referred to as 'light sleep' because we're half awake and half asleep. During this phase our muscle activity slows down and slight twitching may occur. Though we are technically sleeping, we can still be easily roused.
Around ten minutes into the 'light sleep’ stage we begin to move into stage two, which lasts around 20 minutes. In this stage our breathing pattern and heart rate start to slow down. This period accounts for the largest part of human sleep.
During stage three, our brain begins to produce delta waves, a type of wave that is large (high amplitude) and slow (low frequency). Our breathing and heart rate slow to their lowest levels during this part of the sleep cycle.
After this we enter stage four, the final phase of non-REM sleep which is characterised by a combination of limited muscle activity and rhythmic breathing. If we are awakened during this deep sleep we do not adjust immediately and often feel groggy and disoriented for several minutes after waking up. Some children experience bed-wetting, night terrors, or sleepwalking during this stage.
During non-REM sleep the body has the opportunity to fix any wear and tear from throughout the day, repairing and regenerating tissue, building muscle and bone and strengthening the immune system.
Approximately 25 per cent of the sleep cycle is spent in REM sleep. The first rapid eye movement (REM) period usually begins about 70 to 90 minutes after we fall asleep. Although we are not conscious, the brain is very active - often more so than when we are awake. This is the period when most dreams occur. Our eyes dart around (hence the name) and our breathing rate & blood pressure rise. Despite the increased activity in the brain, our muscles remain paralysed, which is thought to be the body's way of preventing us from acting out our dreams.
We experience between three and five REM episodes each night and after each REM sleep the whole cycle begins again. As the night progresses each cycle becomes less dominated by the non-REM phases and progressively more dominated by REM sleep.
Our dreams can occur throughout any sleep stage but the most vivid dreams tend to be reported when people are awoken from REM sleep.