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Sleep spindles, marked by spindle-like changes in the amplitude of 12–14 Hz oscillations, K complexes lasting at least 0.5 seconds, consisting of a distinct negative sharp wave followed by a positive component, and slow waves or delta waves characterized by slow frequency (< 2 Hz) and high amplitude (> 75 μV) are key indicators. [21]
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.
Slow-wave sleep is thought to be the most restful form of sleep, the phase which most relieves subjective feelings of sleepiness and restores the body. [76] This stage is characterized by the presence of a minimum of 20% delta waves ranging from 0.5–2 Hz and having a peak-to-peak
Gamma waves are seen when a person is highly focused on a task or using all their concentration. Theta waves occur during the period of a person being awake, and they continue to transition into Stage 1 of sleep and in stage 2. Delta waves are seen in stages 3 and 4 of sleep when a person is in their deepest of sleep. [17]
The activity of K-complexes is transferred to the thalamus where it synchronizes the thalamocortical network during sleep, producing sleep oscillations such as spindles and delta waves. [7] It has been observed that they are indeed identical in the "laminar distributions of transmembrane currents" to the slow waves of slow-wave sleep. [1]
The time scale at which the waves travel at is the same speed it takes for neurons to communicate with each other. [8] Doubts, however, remain whether a link exists between sleep spindles and memory with a recent meta-review of 53 studies concluding that "there is no relationship between sleep spindles and memory, and thus it is unlikely that ...
Delta wave activity, correlating with slow-wave (deep) sleep, in particular shows regular oscillations throughout a good night's sleep. Secretions of various hormones, including renin, growth hormone, and prolactin, correlate positively with delta-wave activity, while secretion of thyroid-stimulating hormone correlates inversely. [3]
Tonic REM is characterized by theta rhythms in the brain; phasic REM is characterized by PGO waves and actual "rapid" eye movements. Processing of external stimuli is heavily inhibited during phasic REM, and recent evidence suggests that sleepers are more difficult to arouse from phasic REM than in slow-wave sleep. [18]