Quantitative topographic EEG data were collected from 130 subjects during eyes closed and eyes open baseline conditions between 8 am and 8 pm. Two hundred and seventy records per condition were analyzed in reference to time of day at data acquisition for 1-Hz
frequency bands from 1 to 24 Hz.
Circadian and
ultradian rhythms were evident in all frequency bands, most notably during the eyes closed condition. Peak amplitude varied as a function of time of day and frequency band. Alpha
activity peaked in mid-afternoon
hours whereas activity in lower (e.g., theta) and higher (e.g., beta) frequencies peaked during morning hours. Activity between 7-12 Hz exhibited a relatively distinct and fast ultradian
rhythm, imposed on a less prominent circadian cycle, whereas higher and lower frequency ranges exhibited a slower semicircadian rhythm only. Intermediate activity (9-11Hz), associated with cognitive integration, showed a 100-120 min ultradian contribution similar in its periodicity to the Basic-Rest-Activity-Cycle of Kleitman. The presence or absence of an ultradian rhythm may reflect the homogeneity of a signal's source; whether one or more mechanisms (or networks) are involved in a frequency band's generation. Spectral
modulation was cued more to clock time than sidereal time, suggesting that human cycles of activity, wakefulness, and eating influenced circadian modulation more than daylight hours. The relevance of these preliminary findings to neurofeedback training will be discussed.