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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
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 (SWS), often referred to as deep sleep, is the third stage of non-rapid eye movement sleep (NREM), where electroencephalography activity is characterised by slow delta waves. [ 2 ] Slow-wave sleep usually lasts between 70 and 90 minutes, taking place during the first hours of the night. [ 3 ]
Theta and delta waves are not generally seen in wakefulness - if they are, it is a sign of brain dysfunction. [ 3 ] EEG can detect abnormal electrical discharges such as sharp waves , spikes, or spike-and-wave complexes, as observable in people with epilepsy ; thus, it is often used to inform medical diagnosis .
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 .
A fictional EEG showing a sleep spindle and K-complex in stage 2 sleep.. Sleep spindles are bursts of neural oscillatory activity that are generated by interplay of the thalamic reticular nucleus (TRN) and other thalamic nuclei during stage 2 NREM sleep in a frequency range of ~11 to 16 Hz (usually 12–14 Hz) with a duration of 0.5 seconds or greater (usually 0.5–1.5 seconds).
Activity of the neocortex during slow wave sleep determines inputs to the hippocampus; thalamocortical sleep spindles and delta waves are the sleep patterns of the neocortex. [12] These inputs contribute to the selection of different neuronal assemblies for initiation of SWRs, and affect the timing of the SWRs. [4]
A delta wave is an initial slurred deflection seen in the initial part of an otherwise narrow QRS of a patient at risk for WPW and is an indicator of the presence of an accessory pathway. These beats are a fusion between the conduction down the accessory pathway and the slightly delayed but then-dominant conduction via the AV node.