Short and long ultradian EEG components in daytime arousal

In order to investigate the spatio-temporal organization of the daytime arousal state, polygraphic recordings were obtained on 9 subjects, during the period from 11.00 a.m. to 19.30 p.m. Multiple sleep latency tests (MSLTs) administered every 20 min were used as an objective index of sleepiness. Long-duration, wide-band EEG was analyzed by principal components analysis (PCA) to extract the features of ultradian rhythmicities. The data matrix for PCA was constructed from the percentage-power values of 16 frequency bands (2-18 Hz). The diurnal rhythm of the arousal EEG was made up of 2 ultradian components with periods of about 100 min and 3-8 h. The shorter component is thought to represent the oscillation of vigilance level between mental 'rest' and 'activity' states in the basic rest-activity cycle (BRAC). The longer component is thought to represent variations in levels of consciousness between 'wakefulness' and 'drowsiness' states. Our interpretation of the data suggests that the shorter component is superimposed upon the longer one, and that the specific arousal state involved in the shorter ultradian rhythmicity changes to another state at around the 'breaking point' in mid-afternoon.