Physical exercise at night blunts the nocturnal increase of plasma melatonin levels in healthy humans.

The effects of physical exercise on nighttime melatonin secretion have never been investigated in humans. For this purpose, plasma melatonin levels were measured at different times during the day and the night in seven healthy men (aged 26-33 yrs), both in resting condition and before and after a physical exercise performed between 10.40 and 11.00 p.m.. The exercise consisted in bicycling on a bicycle ergometer at 50% of the personal maximal work capacity (MWC) for 10 min, followed by other 10 min of bicycling at 80% of the MWC. The results clearly showed that physical stress at night significantly blunts the nocturnal increase in plasma melatonin levels (group X time interaction: p less than 0.00001; two-way ANOVA with repeated measures). These findings, taken together with the data of the literature, suggest that the response of the pineal gland to provocative stimuli may depend on its level of activity when the stimulus is applied.