Melatonin in drinking water influences a circadian rhythm of locomotor activity in the house cricket, Acheta domesticus

The locomotor activity of Acheta domesticus in light–dark (LD) cycles became more synchronous, i.e. it had a smaller variance around the mean, as the concentration of melatonin in drinking water increased. The ratio of night activity to total activity was increased and the acrophase was delayed by melatonin drinking. Bouts of activity also decreased in LD cycles as the concentration of melatonin increased. The number of activity bouts in the free-running rhythm in DD, however, was not influenced by the melatonin concentration. The most apparent effect of melatonin on the activity in LD cycles seems to be the elimination of day-time activity. Not only did melatonin eliminate day-time activity to give a better synchronization in LD cycles but it also affected the free-running rhythm, which tended to be more synchronized, i.e. the activity became more concentrated, as the concentration of melatonin increased. The amplitude of free-running rhythms also increased as the melatonin concentration increased, even at 100 μg melatonin/ml, though the amplitude in DD was lower than that in LD cycles.