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.
How Sleep Affects Your Brain
Technologies for monitoring sleep
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, an EEG can monitor brainwaves throughout sleep, which has revealed that there are different stages of sleep, each of which is characterized by unique brain activity.
Brainwaves in sleep stages
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 a slow movement of the eyes and reduced activity of the voluntary muscles in the body. The brainwaves in stage 1 sleep are smaller and more uniform than in the awake state, which is referred to as alpha and theta waves.
In stage 2 sleep, the movement of the eyes ceases and the brainwaves 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 brainwaves called delta waves. This stage of sleep is very heavy with no movement of the eyes or voluntary muscles. Additionally, it is often difficult to wake a person in this stage of sleep.
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 (CNS).
REM brain activity
It is believed that dreaming occurs for at least 2 hours each night during REM sleep and that this activity plays an important role in the processing of information and the 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 their days sleeping (up to 50%) in comparison to adults, thus leading to the hypothesis that brain activity during sleep helps in the development of 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 the processes of learning, thinking, and organizing information.
Sleep stage cycles
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, followed by stages 2 and 3 for about 10 and 30 minutes, respectively. Finally, the individual will reach REM sleep more than one hour after first falling asleep.
Shortly after, the individual returns to stage 2 sleep, then stage 3 sleep, and will ultimately reach REM sleep once again, with this cycle repeating approximately five times before awakening.
It is unclear why this cycling through the stages of sleep and continual changes in 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 brain activity.