In view of the hypothesis that adenosine is involved in sleep regulation, the effects of the adenosine antagonist caffeine on sleep and sleep EEG were investigated in eight young males. Compared to the placebo condition, caffeine (100 mg) administered at bedtime prolonged sleep latency and reduced sleep efficiency and stage 4 of non-rapid eye movement sleep (NREMS). Electroencephalographic slow-wave activity (SWA, spectral power density in the 1.75-4.5-Hz band) was reduced, whereas power density in the spindle frequency range was slightly enhanced. The suppression of SWA was limited to the first NREMS episode. Caffeine reduced the power density mainly in the lowest delta band, in contrast to the changes during physiological sleep that encompass both the delta and theta bands. Caffeine levels in saliva, assessed in a separate experiment, decreased from 7.5 mumol/l in the first hour of sleep to 3.5 mumol/l in the seventh hour. In the night following caffeine administration, stage 4 sleep had reverted to the baseline level, but sleep latency was still increased, and stage 2 sleep, as well as SWA in the first NREMS episode, were reduced. The data show that even a low dose of caffeine affects the sleep EEG. However, the effects of caffeine did not completely mimic the spectral changes observed during physiological sleep.