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.