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The effect of a moderate dose of ethanol (0.55 g/kg of body weight), administered 6 hours before scheduled bedtime, on performance, nocturnal sleep, and the sleep electroencephalogram (EEG) was investigated in 10 healthy, middle-aged men (mean age: 61.6 +/- 0.9 years). By the beginning of the sleep episode, breath-ethanol concentrations had declined to zero in all subjects. Compared with the control condition (mineral water), sleep was perceived as more superficial. Sleep efficiency, total sleep time, stage 1, and rapid eye movement (REM) sleep were reduced. In the second half of the sleep episode, wakefulness exhibited a twofold increase. EEG power density in low delta frequencies was enhanced in non-REM sleep (1.25-2.5 Hz) and REM sleep (1.25-1.5 Hz). In slow wave sleep (i.e., stages 3 + 4), power density was increased not only in the low-frequency range (1.25-1.5, 2.25-4.0, 4.75-5.0 Hz) but also within the alpha (8.25-9.0 Hz) and sigma (12.25-13.0 Hz) band. The data demonstrate that late-afternoon ethanol intake in middle-aged men disrupts sleep consolidation, affects the sleep stage distribution, and alters the sleep EEG. (J Clin Psychopharmacol 1996;16:428-436).