Objective

Melatonin, a hormone released preferentially by the pineal gland during the night, affects circadian rhythms and aging processes. As animal studies have shown that melatonin increases resident-intruder aggression, this study aimed to investigate the impact of melatonin treatment on human aggression.

Methods

In a double-blind, randomized, placebo-controlled between-participant design, 63 healthy male volunteers completed the Taylor Aggression Paradigm (TAP) after oral administration of melatonin or placebo.

Results

We found that when given the opportunity to administer high or low punishments to an opponent, participants who ingested melatonin selected the high punishment more often than those who ingested placebo. The increased reactive aggression under melatonin administration remained after controlling for inhibitory ability, trait aggression, trait impulsiveness, circadian preference, perceptual sensibility to noise, and changes in subjective sleepiness and emotional states.

Conclusion

This study provides novel and direct evidence for the involvement of melatonin in human social processes.