The circadian covariation of fatigue and urinary melatonin

Twelve healthy male subjects were kept under constant conditions (no sleep, isolation from time cues, controlled activity, etc.) for 64 hr. Urinary melatonin values, self-rated sleepiness, and vigilance performance scores were obtained every 3 hr. All variables showed a pronounced circadian rhythmicity. Vigilance performance and self-rated sleepiness showed, in addition, a gradual decrease and increase, respectively, with increasing sleep deprivation. The correlation of melatonin with performance and ratings was highly significant, high melatonin levels being associated with reduced performance and increased sleepiness. Aligning self-ratings and behavioral data with respect to the melatonin troughs and peaks showed that the former coincided with performance and alertness peaks and the latter with the troughs. It was concluded that under these conditions there is a strong circadian covariation between melatonin and indices of fatigue/sleepiness.