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REM sleep is characterized by the lack of muscle activity. Physiological studies have shown that aside from the occasional twitch, a person actually becomes paralyzed during REM sleep. [7] In motor skill learning, an interval of sleep may be critical for the expression of performance gains; without sleep these gains will be delayed. [8]
Sharp waves and ripples (SWRs) are oscillatory patterns produced by extremely synchronised activity of neurons in the mammalian hippocampus and neighbouring regions which occur spontaneously in idle waking states or during NREM sleep. [1] They can be observed with a variety of imaging methods, such as EEG.
Sleep stages are characterized by spectral content of EEG: for instance, stage N1 refers to the transition of the brain from alpha waves (common in the awake state) to theta waves, whereas stage N3 (deep or slow-wave sleep) is characterized by the presence of delta waves. [107] The normal order of sleep stages is N1 → N2 → N3 → N2 → REM.
Neurophysiological studies have indicated a relationship between increased P-wave density during post-training REM sleep and learning performance. [20] [21] Basically, the abundance of PGO waves translates into longer periods of REM sleep, which thereby allows the brain to have longer periods where neuronal connections are formed.
In older children and adults, it tends to appear during meditative, drowsy, hypnotic or sleeping states, but not during the deepest stages of sleep. Theta from the midfrontal cortex is specifically related to cognitive control and alterations in these theta signals are found in multiple psychiatric and neurodevelopmental disorders. [15]
NREM sleep itself is divided into multiple stages – N1, N2 and N3. Sleep proceeds in 90-minute cycles of REM and NREM, the order normally being N1 → N2 → N3 → N2 → REM. As humans fall asleep, body activity slows down. Body temperature, heart rate, breathing rate, and energy use all decrease. Brain waves slow down.
All sleep, even during the day, is associated with the secretion of prolactin. [13] Key physiological methods for monitoring and measuring changes during sleep include electroencephalography (EEG) of brain waves, electrooculography (EOG) of eye movements, and electromyography (EMG) of skeletal muscle activity.
It is most readily observed in stages of sleep, for example, rapid eye movement sleep (REM) and the delta activity cycle. When awake, brainwaves are faster during the first half of the cycle which corresponds to feeling alert and focused. During the last 20 minutes brainwaves slow and as the body feels dreamy or tired.