Slow Wave Sleep (SWS) is a term used to describe stages 3 and 4 sleep.

Electroencephalographic Characteristics

Large 75 microvolt (2 Hz) delta waves predominate the electroencephalogram (EEG). Stage 3 is defined by Rechtschaffen and Kales as the point where delta waves make up 20-50% of a 30 second EEG epoch (generally 8-13 delta waves). Stage four is defined as a 30 second epoch that consists of greater than 50% delta waves (usually 14 or more delta waves per 30 second epoch).

Longer periods of slow wave sleep occur in the first part of the night. Children and young adults will have more total slow wave sleep in a night than older adults. The elderly may not go into slow wave sleep at all during many nights of sleep.

Slow wave sleep is an active phenomenon probably brought about by the activation of serotonergic neurons of the raphe system.

The slow wave seen in the cortical EEG is generated through thalamocortical communication through the thalamocortical neurones. In the TC neurones this is generated by the 'slow oscillation' and is dependent on membrane potential bistability, a property of these neuroes due to a electrophysiological component known as I t Window. I t Window is due to the overlap underneath activation/ inactivation curves if plotted for T-type calcium channels (inward current). If these two curves are multiplied, and another line suprimposed on the graph to show a small Ik leak current (outward), then the interplay between these inward (It Window) and outward (small Ik leak), three equilibrium points are seen at -90, -70 and -60mv, -90 and -60 being stable and -70 unstable. This property allows the generation of slow waves due to an oscillation between two stable points. It is important to note that in vitro, mGluR must be activated on these neurones to allow a small Ik Leak, as seen in in vivo situations.

Characteristics

SWS is often referred to as deep sleep. This is probably due to the often intense groggy feeling one gets when awoken from this stage. If deprived of sleep, there is a sharp rebound of SWS, suggesting a "need" for this stage.

References

Rechtschaffen, A.& Kales, A. "A Manual of Standardized Terminology, Techniques and Scoring System For Sleep Stages of Human Subjects." US Dept of Health, Education, and Welfare; National Institutes of Health. Maryland: 1968.

Sleep | Electroencephalography

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