Alcohol intake alters melatonin secretion both in healthy volunteers and in alcoholics in a variety of different situations (while drinking, during or after withdrawal, and with neurological complications). This alteration may reduce secretion or affect its circadian rhythm, thus causing daytime secretion in some alcoholics. We sought to determine if daytime melatonin secretion is caused directly by acute alcohol consumption or if it instead indicates a change in circadian synchronization. Because alcohol consumption as it occurs in alcoholics (continuous consumption of large amounts) has never been examined in healthy volunteers, we exposed 11 healthy volunteers to 256 g of alcohol over 24 h to study the circadian profiles of melatonin secretion. Our results demonstrate a lack of daytime secretion in our subjects. This suggests that the disordered circadian melatonin secretion seen in alcoholics indicates a shift in melatonin secretion rather than an acute effect of alcohol on this secretion, or alternatively, that it is a direct effect of chronic rather than acute exposure to high blood alcohol levels.