EEG power variations were evaluated in 9 healthy young adults from 8.45 a.m. to 9 p.m. and at 7 a.m. the next day. EEG signals were obtained from 16 electrodes in closed eyes and open eyes situations. Diurnal power variations were calculated for each frequency component, according to the recording situation (RS) and to the scalp site. Regarding values in the early morning on the first day, the power of almost all the frequency components showed an important diurnal increase. It came back close to initial values at 7 a.m. on the second day, which is in agreement with the existence of EEG circadian variations. Diurnal evolutions were dependent on the frequency components: the higher the frequency, the later was its diurnal maximum. For many frequency components, the diurnal variation was dependent on RS and the scalp topography. All these characteristics could be used to split the classical EEG bands, especially the delta and alpha bands and be useful for physiological and pharmacological research.