Effect of Melatonin on the Human Electrocardiogram and Simple Reaction Time Responses

It has been suggested that melatonin may have therapeutic potential for alleviating jet lag. In an attempt to define further the effects of low pharmacological doses of melatonin on the human body, 5 mg of melatonin was given orally to one subject in a blind experiment. The electrocardiogram (ECG) was recorded together with simple reaction time (SRT) responses over a 5-h period after taking melatonin. The main part of the project was carried out under artificial domestic lighting conditions and involved ten evening sessions. Microprocessor analysis of the ECG of one subject clearly showed that melatonin had the effect of lengthening the RR interval, that between the peak of the P wave and the onset of the R wave, and the interval between the onset of the R wave and the peak of the T wave. A model to explain the cubic curve, which was fitted by regression analysis to the data, implies that the natural evening increase in melatonin may also increase these intervals. The results suggest that melatonin may affect both the phase and the amplitude of the observed ECG phenomenon. Analysis of more than 40,000 SRT responses to both visual and auditory stimuli implied that these responses might also be lengthened by melatonin. Frequency histograms of the responses and observations on premature and missed responses were not able to reproduce the results of other workers who found that higher doses (240 mg) of melatonin decreased the number of incorrect responses to simple reaction time tasks.