Lack of exercise disrupts body’s rhythms.

Natural daily rhythms in spontaneous movement patterns in both humans and mice show scale invariance, i.e., movement patterns repeat over time scales of minutes to hours. These scale invariant patterns decay with aging in both humans and mice, apparently correlating with progressive dysfunction of circadian pacemaker circuits in the brain's suprachiasmatic nucleus. Scheer and collaborators have now shown that in both aged and young mice exercise is a crucial variable. Loss of scale invariance associated with both inactivity and aging can be restored by exercise, even in old animals.

In healthy humans and other animals, behavioral activity exhibits scale invariance over multiple timescales from minutes to 24 h, whereas in aging or diseased conditions, scale invariance is usually reduced significantly. Accordingly, scale invariance can be a potential marker for health. Given compelling indications that exercise is beneficial for mental and physical health, we tested to what extent a lack of exercise affects scale invariance in young and aged animals. We studied six or more mice in each of four age groups (0.5, 1, 1.5, and 2 y) and observed an age-related deterioration of scale invariance in activity fluctuations. We found that limiting the amount of exercise, by removing the running wheels, leads to loss of scale-invariant properties in all age groups. Remarkably, in both young and old animals a lack of exercise reduced the scale invariance in activity fluctuations to the same level. We next showed that scale invariance can be restored by returning the running wheels. Exercise during the active period also improved scale invariance during the resting period, suggesting that activity during the active phase may also be beneficial for the resting phase. Finally, our data showed that exercise had a stronger influence on scale invariance than the effect of age. The data suggest that exercise is beneficial as revealed by scale-invariant parameters and that, even in young animals, a lack of exercise leads to strong deterioration in these parameters.