Colliding of activity onset and offset: evidence for multiple circadian oscillators.

When hamsters, Mesocricetus auratus, are kept in dim light, wheel running at the onset and the offset of their active phase have different circadian periods. As a result, the active phase expands and eventually the two activity components collide. There is then a temporary explosion of activity at a time that was previously in the rest period. This is followed by disorganization of the rhythm or by phase jumps. Such phase jumps probably stem from the interaction of two oscillators with different natural periods rather than from threshold changes for the expression of activity.