The rapid eye movement (REM) stage of sleep is defined by characteristic REMs, low-amplitude electromyographic activity, and low-voltage mixed frequency EEG background activity (Rechtschaffen and Kales, 1968). The mixed frequencies predominantly include activity in the theta frequency range and the alpha frequency range at a frequency that is 1 to 2 Hz slower than the individual’s alpha rhythm (Rama et al., 2009). Bursts of slower activity also characterize stage REM. These bursts, called saw-tooth waves, are regular 2 to 6 Hz activity over the midline (Fisch, 1999; Silber et al., 2007). The name saw-tooth refers to a waveform for the individual waves comprising a burst of successive waves in a repetition. These waves are monophasic and triangular with a rise and fall that is increasingly steep around the wave’s apex and this gives the characteristically sharp appearance. In addition, superimposed faster frequencies sometimes give the appearance of serration (Blume et al., 2002). The waveform is best appreciated with an epoch time (display window) of 30 seconds, which is a standard page duration used for polysomnograms and more compressed than routine EEG’s usual page duration of 10 to 12 seconds.

The 2 to 6 Hz frequency range for the individual saw-tooth waves within a burst spans the breadth of saw-tooth wave frequencies that have been described and includes both theta and delta frequencies (Pearl et al., 2002; Yasoshima et al., 1984). The bursts have a total duration that varies from about 2 to 26 seconds with an average duration of approximately 7 seconds. Bursts recur during stage REM approximately once per minute (Pearl et al., 2002). Although saw-tooth waves originally were described as having a frontal or central location, a more recent investigation found them to be consistently maximal at the vertex with a distribution that decreases equally in both the anterior and posterior directions (Broughton and Hasan, 1995; Yasoshima et al., 1984). Essentially, their field is similar to that of vertex sharp transients and K complexes. Saw-tooth wave amplitude is usually less than 50 μV, but it can vary from 20 to 100 μV.