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The sleep cycle consists of four stages. Non-rapid eye movement (NREM) sleep covers the first three stages, wherein the sleeper starts from light sleep and enters deep sleep. Rapid eye movement (REM) sleep is the last stage of the sleep cycle. So how are NREM and REM sleep different and how is REM sleep important?
What is REM sleep?
In contrast to the slow wave activity found in Stage 3 of NREM sleep, REM sleep is defined by small (low amplitude) and fast (high frequency) brainwave activity. REM sleep is regulated by electrical and chemical activity that appears to originate from the brain stem. During this stage of sleep, the body is almost completely paralysed (atonia). Despite this, the brain is comparably just as active as during wakefulness. In fact, the amount of energy consumed by the brain during REM sleep is equal to or even exceeds the energy used when awake. The neurotransmitter acetylcholine is produced in large amounts during REM sleep, which may be cause faster brainwaves and induce the transition from slow wave sleep to REM sleep.
What happens during REM sleep?
REM sleep can also be characterised by vivid dreams and, as its name indicates, rapid eye movement. Compared to NREM sleep, dreams in REM sleep have a convincing resemblance to experiences during wakefulness. When awoken from REM sleep, sleepers tend to give longer and more descriptive accounts of their dreams, and tend to estimate the length of their dreams to last longer. Despite being a distinctive feature of REM sleep, the purpose behind eye movements is still uncertain. It is suggested that the eye movements may be associated with dreaming. However, congenitally blind people have eye movements during REM sleep as well, even though they do not typically experience visual imagery when dreaming. Alternatively, eye movements may be a side effect of the brain processing eye-related aspects of memory.
The body temporarily withholds homeostatic regulation during REM sleep. Homeostasis is the maintenance of stable internal, physical, and chemical equilibrium within a living being. Functions such as breathing rate and heart rate become irregular during REM sleep. The internal regulation of body temperature is also interrupted. The ability to thermoregulate through physical means (e.g. shivering) is lost due to the paralysation of the body, and neurons that typically respond to cold temperatures do not activate during REM sleep. Once REM sleep ends, homeostatic regulation returns.
Why is REM sleep necessary?
Depriving a living organism from REM sleep can be detrimental to their survival, and a prolonged loss can lead to death. This strongly suggests that REM sleep is vitally responsible for addressing the body’s physiological needs, although what factors and how REM sleep affects them is not quite well understood yet. Given its variance with age, REM sleep is speculated to be important for brain development. Infants and children spend significantly more of their sleep time in REM sleep compared to adults. It is theorised that REM sleep helps the developing brain form neural connections by providing neural stimulation. REM sleep deprivation in the early years of life can lead to behavioural issues, permanent sleep disruption, and diminished brain mass. Like NREM sleep, REM sleep has an important role in processing memory, although different types of memory seem to be processed at different stages of the sleep cycle. Experiments have demonstrated that rats after intensive training to complete a task experienced increased REM sleep, while REM sleep deprivation can impair memory consolidation.
Hobson, J., Pace-Schott, E., & Stickgold, R. (2000). Dreaming and the brain: Toward a cognitive neuroscience of conscious states. Behavioral and Brain Sciences, 23(6), 793-842.
Parmeggiani, P. L. (2011). Systemic homeostasis and poikilostasis in sleep: Is REM sleep a physiological paradox?. World Scientific.
Rasch, B., & Born, J. (2013). About Sleep’s Role in Memory. Physiological Reviews, 93(2), 681-766.