Behavioral Decoupling of Circadian Rhythms

Golden hamsters (Mesocricetus auratus) were kept in a light-dark cycle (LD 14:10). For 2 weeks, almost every day they were placed in a novel running wheel for 3 hr, starting 7 hr before dark onset. Most of the animals made several thousand wheel revolutions during this 3 hr. When these animals were subsequently transferred to a dark room, their activity was split into two components, one close to the time of the previous exposure to the novel wheel and the other close to the time when they had been active in the dark phase of the previous LD cycle. The two components fused after a few days in darkness. These observations show that nonphotic events are capable of causing major reorganizations of circadian activity patterns, despite the presence of an LD cycle.