Figure 1.

Age-related changes in the circadian pattern of behavioral activity. A, Representative plots showing 9 d of wheel-running activity in DD in young (100 d), middle-aged (500 d), and old (900 d) mice. Data are double plotted for clarity. B, The cycle length in DD (τ) shows a marked increase between 100 and 300 d old and remains constant at later ages. C, The amount of daily activity decreases between 100 and 300 d. D, The duration of the resting phase (ρ) is constant in the first three age groups and increases at 700 d. E, The duration of the active phase (α) decreases at 700 d. The duration of α and ρ are plotted as a fraction of the circadian cycle (τ/24) to correct for the age-related period lengthening. F, Longitudinal analysis of individual running wheel records (n = 5 animals; colored markers represent the average activity bout duration during 7 d) shows that both the duration of consolidated activity bouts and time spent in the wheel declines as a function of age. All parameters in B–E show significant changes with age.