Effect of daytime oral melatonin administration on neurobehavioral performance in humans

The pineal hormone melatonin has been proposed as a potential treatment for insomnia and circadian rhythm disorders, as experienced by shiftworkers and transmeridian travellers. If melatonin is to be used in a clinical setting, it is important that the neurobehavioral performance effects associated with its administration be identified. As with other drugs that increase sleepiness, e.g., sedatives and antihistamines, inappropriate effects on neurobehavioral performance could result in a corresponding increase in side effects and potential risk to users. The aim of this study was to investigate the effects of 5 mg oral melatonin, administered at 12.30 hr to 16 young (22.4 +/- 1.8 years), healthy subjects (ten males and six females), on the following neurobehavioral performance tasks: two-choice visual reaction and response time, unpredictable tracking, extended two-choice visual reaction and response time, and simple auditory response and reaction time, in a randomised, double-blind crossover protocol. Following melatonin administration, significant decrements in performance on the tracking task and on response and reaction time scores for the visual choice and extended two-choice visual tasks were observed. The profile of these performance decrements across the experimental sessions were found to closely map the profile of salivary melatonin levels. The findings of the present study highlight the significant neurobehavioral performance effects associated with the daytime administration of melatonin.