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One of the important questions in sleep research is clearly defining the sleep state. This problem arises because sleep was traditionally defined as a state of consciousness and not as a physiological state, [14] [15] thus there was no clear definition of what minimum set of events constitute sleep and distinguish it from other states of partial or no consciousness.
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]
Sleep deprivation also has a documented effect on the ability to acquire new memories for subsequent consolidation. A study done on mice that were sleep deprived before learning a new skill but allowed to rest afterward displayed a similar number of errors on later trials as the mice that were sleep deprived only after the initial learning. [46]
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.
Sharp waves and associated ripples are present in the mammalian brains of the species that have been investigated for this purpose, including mice, rats, rabbits, monkeys and humans. [5] In all of these species, they have been shown primarily to be involved in consolidation of recently acquired memories during the immobility and slow-wave sleep.
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.
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]
Young woman asleep over study materials. The relationship between sleep and memory has been studied since at least the early 19th century.Memory, the cognitive process of storing and retrieving past experiences, learning and recognition, [1] is a product of brain plasticity, the structural changes within synapses that create associations between stimuli.