Two circadian rhythms in the human electroencephalogram during wakefulness

The influence of the circadian pacemaker and of the duration of time awake on the electroencephalogram (EEG) was investigated in 19 humans during simi lar to 40 h of sustained wakefulness. Two circadian rhythms in spectral pow er density were educed. The first rhythm was centered in the theta band (4. 25-8.0 Hz) and exhibited a minimum similar to 1 h after the onset of melato nin secretion. The second rhythm was centered in the high-frequency alpha b and (10.25-13.0 Hz) and exhibited a minimum close to the body temperature m inimum. The latter rhythm showed a close temporal association with the rhyt hms in subjective alertness, plasma melatonin, and body temperature. In add ition, increasing time awake was associated with an increase of power densi ty in the 0.25- to 9.0-Hz and 13.25- to 20.0-Hz ranges. It is concluded tha t the waking EEG undergoes changes that can be attributed to circadian and homeostatic (i.e., sleep-cake dependent) processes. The distinct circadian variations of EEG activity in the theta band and in the high-frequency alph a band may represent electrophysiological correlates of different aspects o f the circadian rhythm in arousal.