OBJECTIVE: Daytime melatonin ingestion is known to induce sleep at rest, which may affect arousal and vigilance. Physical exercise is known to produce an increase in core temperature and circulating cortisol which can enhance arousal and vigilance. The effect of submaximal exercise on vigilance and arousal following acute melatonin ingestion has not yet been studied. The present study aimed at investigating the effect of submaximal exercise on vigilance and arousal following daytime melatonin ingestion. PATIENTS AND METHODS: Eight physical education students undertook 45 min of submaximal exercise (at 60% of maximal aerobic speed) on a treadmill after melatonin-(6 mg) or placebo ingestion, in a randomized and counterbalanced order. RESULTS: Heart rate (HR), rectal temperature (Tre), felt arousal scale (FAS), and thermal sensations (TS) were recorded at baseline (pre-exercise), immediately after exercise (post-exercise), and after 30 min of recovery (30 min post-exercise). Blood was sampled for lactate and cortisol. At 30 min post-exercise, the Tre, HR, blood pressure, lactate, FAS, and TS were measured. The participants performed vigilance tests pre-exercise, post-exercise and 30 min post-exercise. Daytime melatonin ingestion affected arousal and vigilance in the pre-exercise period (p < 0.05) but had no effect on Tre, HR, blood pressure, lactate, TS, arousal, and vigilance measured 30 min post-exercise (p > 0.05). CONCLUSIONS: The negative effects of melatonin ingestion on vigilance and arousal vanished after a 45 min of submaximal exercise. The hypnotic effect of melatonin observed in the pre-exercise dissipated in the post-exercise period, possibly due to the significant elevation of Tre, HR, and cortisol at the end of submaximal exercise.
