Loss of the circadian rhythms of locomotor activity, food intake, and plasma melatonin concentration induced by constant bright light in the pigeon (Columba livia).

Locomotor activity and feeding activity were measured together with circulating levels of melatonin in pigeons which were exposed to constant bright light (LLbright, 2000 lux) following light-dark (LD) cycles. Although all the pigeons showed daily rhythms of locomotor activity, feeding activity, and melatonin levels under LD cycles, they lost all the rhythms in prolonged LLbright. Acute exposure to bright light (2000 lux) during darkness reduced plasma melatonin levels. The half-time for the suppression in melatonin levels was about 30 min after short-term light exposure. These results support the hypothesis that melatonin may control the circadian rhythms of locomotor activity and feeding activity in the pigeon.