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Although it was historically believed that sleep was a passive but necessary process for healthy bodily functions, it is now known that brain activity continues during sleep. In fact, this brain activity is thought to play several important roles in the maintenance of physical, emotional and mental health.
Sleep research has progressed significantly following the introduction of technology that enables the observation and monitoring of brain activity during sleep. This includes positron emission tomography (PET) scanning, functional magnetic resonance imaging (fMRI) and electroencephalogram (EEG).
In particular, EEG is able to monitor the brain waves throughout sleep, which has revealed that there are different stages of sleep, each of which is characterized by unique brain activity.
The different stages of sleep are known as stage 1, stage 2, stage 3 and rapid eye movement (REM) sleep and can be distinguished by the brainwaves in each of these stages.
Stage 1 sleep is the lightest stage of sleep that occurs as an individual is falling asleep. There is slow movement of the eyes and activity of the voluntary muscles in the body is reduced. The brainwaves in stage 1 sleep are smaller and more uniform than in the awake state, what are referred to as alpha and theta waves.
In stage 2 sleep, the movement of the eyes ceases and the brain waves become slower than in stage 1. There are also occasional bursts of waves that are more rapid, which are referred to as sleep spindles.
Stage 3 of sleep is characterized by slow, rhythmical brain waves called delta waves. This stage of sleep is very heavy with no movement of the eyes or voluntary muscles, and it is difficult to wake a person in this stage.
During REM sleep, an individual usually breathes more rapidly and there are quick movements of the eyes that characterize the state. In this stage, the brain activity according to the EEG is very similar to that of a person who is awake, suggesting that there are significant processes taking place in the central nervous system.
It is believed that dreaming occurs for at least 2 hours each night during REM sleep and this activity plays an important role in the processing of information and creation of memory. During this stage of sleep, heart rate and blood pressure increase and the activity of the brain is markedly more dynamic.
Sleep research with EEG monitoring has established that infants spend a greater proportion of sleep time (up to 50%) in comparison to adults, leading to the hypothesis that the brain activity helps in the development of the memory and learning.
The signals initiate at the base of the brain, in an area referred to as the pons, and then expand to the thalamus and cerebral cortex. The cerebral cortex is responsible for processes of learning, thinking and organizing information.
Over time, an individual progresses through the different stages of sleep and the activity of the brain changes accordingly. It begins with stage 1 for about 5-10 minutes, then stage 2 for about 10 minutes, then stage 3 for about 30 minutes, before reaching REM sleep more than an hour after first falling asleep.
Shortly after, the individual returns to stage 2 sleep, then stage 3 sleep and then REM sleep once again, repeating this cycle approximately five times before awakening.
It is unclear why this cycling through the stages of sleep and continual changes in the brain activity is required for the healthy function of humans and other mammals. Further research in this area is currently being undertaken to understand this area more comprehensively, particularly for the function of the brain activity.