Wheel running sleep in two strains of mice: plasticity and rigidity in the expression of circadian rhythmicity.

The effects of 12--12 h, 6--6 h, 3--3 h and 1--1 h light--dark (L-D) cycles and of constant light (L--L) on wheel-running activity and electrograhically defined sleep were studied in two strains of mice, C57BL/6 and SEC/1Re. Wheel-running activity was inhibited by light and enhanced by darkness; however, in the C57 strain the L-D-induced changes were less pronounced and superimposed to a clear circadian rhythm. Under the L-L schedule clear patterns of daily rhythmicity were evident in the C57 strain but not in the SEC strain. Sleep was enhanced by light and inhibited by darkness. The L-D-induced changes of sleep were superimposed on the circadian rhythm in both strains; however, in the C57 strain these changes were larger in the circadian phase in which the animals were active. Under the L-L-schedule the C57 strain was characterized by more clear-cut patterns of daily rhythmicity than SEC mice. These phenomena are discussed in terms of plasticity of free-running rhythms of sleep and activity and of the genetic factors involved in the expression of circadian rhythmicity.