Abstract
In the mouse, activity is precisely timed by the circadian clock and is normally most intense in the early subjective night. Since vigorous activity (e.g., wheel running) is thought to induce phase shifts in rodents, the temporal placement of daily exercise/activity could be a determinant of observed circadian rhythm period. The relationship between spontaneous running-wheel activity and the circadian period of free-running rhythms was studied to assess this possibility. With ad libitum access to a running wheel, mice exhibited a free-running period (τ) of 23.43 ± 0.08 hr (mean ± SEM). When running wheels were locked, τ increased (23.88 ± 0.04 hr, p < 0.03), and restoration of ad libitum wheel running again produced a shorter period (τ = 23.56 ± 0.06 hr, p < 0.05). A survey of free- running activity patterns in a population of 100 mice revealed a significant correlation between the observed circadian period and the time of day in which spontaneous wheel running occurred (r = 0.7314, p < 0.0001). Significantly shorter periods were observed when running was concentrated at the beginning of the subjective night (τ = 23.23 ± 0.04), and longer periods were observed if mice ran late in the subjective night (τ = 23.89 ± 0.04), F (1, 99) = 34.96, p < 0.0001. It was previously believed that the period of the circadian clock was primarily responsive to externally imposed tonic or phasic events. Systematic influences of spontaneous exercise on τ demonstrate that physiological and/or behavioral determinants of circadian timekeeping exist as well.