Compression and expansion of circadian rhythm in mice under long and short photoperiods.

Although the functional aspects of synchronization of the circadian pacemaker by environmental light have been extensively studied, few studies have provided systematic information about the temporal organization of behavior under light-dark cycles with varying proportions of light and darkness. In the present study, the running-wheel activity profiles of mice were investigated under short, medium, and long photoperiods. The results clearly indicated that the temporal distribution of locomotor activity in mice is modulated by photoperiod. The activity profile was compressed under long photoperiods and expanded under short photoperiods. Although negative masking by light and alterations in the state of dark adaptation may have partially accounted for the phenomenon, the major mechanism seemed to be a compression and expansion of the circadian pacemaker's cycle, as expressed in the compression and expansion of the photic phase-response curve.