Circadian acrophases of powers and frequencies in the waking EEG.

Circadian changes in frequencies and powers of the close-eyed EEG were measured over six days in 12 healthy volunteers living in a regular rest-activity cycle. EEGs were recorded every two hours and the probands were awakened for the recordings during the period of rest. Circadian rhythms occurred in powers and frequencies with diurnal amplitudes differing between individuals and locally on the scalp. Acrophases of peak frequencies and beta-power coincide with those of body temperature between 5 and 7 p.m., however, the crests of the diurnal rhythms in delta-, theta- and alpha-power varied more between individuals and covered systematically different times of the day. Acrophases of delta- and theta-powers occurred during the second half of the rest period and during the first half of the waking period. Only powers in the alpha-band showed locally different acrophases on the scalp. This could be observed in single individuals, too. In the central and parietal leads, acrophases of alpha-power were found during rest, whereas in the occipital leads of some probands acrophases occurred in the second half of the waking period.