A comparison of electroencephalographic frequencies between polysomnograms. This figure compares two 30-second segments of NREM stage 2 sleep obtained in the early morning (3:00 a.m. to 4:00 a.m.) between our patient’s polysomnogram before rapid palatal expansion (RPE; upper panel) and after rapid palatal expansion (lower panel). Seconds are delineated by the vertical dotted lines. The three electroencephalogram channels (F3M2, C3M2, O1M2) in the upper panel (before rapid palatal expansion), demonstrate the presence of a 7 to 11 cycle per second oscillation (alpha frequency; a frequency associated with wakefulness) that is not prominent in the sleep of healthy individuals. In the lower panel (after rapid palatal expansion), this waking alpha frequency is decreased in amplitude (when it is seen) and the predominant frequency is three to five cycles per second (theta frequency; the frequency predominating in NREM stage 1 and NREM stage 2 sleep among healthy individuals). The horizontal red line highlights a section of the two electroencephalograms where the contrasting frequencies are well seen. The decreased presence of the alpha frequency after rapid palatal expansion suggests a decreased state of vigilance between these two polysomnogram segments.