Early morning melatonin administration impairs psychomotor vigilance

The acute soporific effect of melatonin in humans has been demonstrated in a range of studies. How alertness and performance are changed after melaton in given in the morning is not yet known. In a double-blind. placebo-contro lled study of nine healthy young men, melatonin was given at 0700 h under c ontrolled conditions of a modified constant routine protocol lasting 56 h ( 2 days, 3 nights with sleep). A clear decrement in neurobehavioral function s as measured by the Psychomotor Vigilance Test lasted for 6 h after melato nin administration (particularly in the lapse domain and median of the reac tion time) without any effect on a letter cancellation task. A subjective s oporific effect was present but less pronounced. Thus, melatonin taken in t he morning requires caution in situations when high attention is needed. (C ) 2001 Elsevier Science B.V. All rights reserved.