The sedative and performance-disruptive effects of ethanol as a functi
on of basal level of sleepiness/alertness and circadian time was studi
ed. Ethanol (0.5 g/kg) or placebo was administered to 12 healthy, norm
al-sleeping men, aged 21 to 45 years, at two different times of the da
y (09.00 and 16.00 h) after 8 h time in bed (TIB) the previous night a
nd at 16.00 h after 4 h TIB. The Multiple Sleep Latency Test (MSLT) wa
s conducted at 1, 3, 5, and 7 h post-ethanol consumption in a day drin
king and in two evening drinking conditions. Auditory vigilance and di
vided attention performance was assessed 1 and 5 h post-ethanol. A sig
nificant difference in MSLT scores was found between the 4-h and 8-h T
IB conditions reflecting increased sleepiness. The significant reducti
on in MSLT scores after morning ethanol was not seen in either of the
evening ethanol conditions (8-h or 4-h TIBs). Performance on the audit
ory vigilance task 1 h post-administration was disrupted by ethanol co
nsumption in the morning, but not in the evening conditions (8-h or 4-
h TIBs).