The role of period and phase in interactions between feeding- and light-entrainable circadian rhythms

Sixteen blind male rats were maintained on ad lib food and water for 115 days to obtain stable free-running rhythms. Fifteen rats were then exposed to restricted feeding (RF, 4 hr/cycle) for 52 days and the period of food access (T) differed from the period of the free-running rhythm (tau) by 0.05 to 0.30 hr. Among 15 animals exposed to RF, deceleration of the free-running rhythm was observed in 11 rats, small accelerations occurred in 2 rats, and tau remained unchanged in 2 rats exposed to RF and one ad lib rat. The free-running rhythm assumed the period of RF in 3 rats where T-tau was less than 0.1 hr and the change in tau persisted for 42 days after RF. However, other rats failed to synchronize their free-running rhythm despite small period differences. Although these results show that the two underlying circadian pacemaking systems are not functionally independent, they appear to be only weakly coupled.