All-night spectral analysis of the sleep EEG was used to study the effect and after-effect of a single bedtime dose of flunitrazepam (2 mg), flurazepam (30 mg) or triazolam (0.5 mg) in healthy young males. In the night of drug treatment all 3 hypnotics reduced the EEG slow waves and enhanced the activity in the frequency range of sleep spindles. In the subsequent drug-free night, the latter changes were no longer present after triazolam, whereas the reduction of slow wave activity persisted after all compounds.