Effects of age on nocturnal activity rhythms in rats.

Running wheel data were analyzed to examine changes in nocturnal activity rhythms with age in Long Evans rats. Five male animals 30 days of age, the same five rats at 70 days of age, and five older animals at 13 months were given free access to activity wheels in a colony room maintained on an LD12:12 cycle. All subjects demonstrated enhancement of activity during the dark phase of the 24-h cycle. A statistically significant decrease in activity of old rats in comparison with weanling rats was observed only in the dark phase (P less than .0001). The age difference was marginal in the light phase compared to that in the dark phase. However, a marked increase in percentage of time spent active in the light phase was observed in the old animals. This may be a combined result of visual hyposensitivity and insomnia that became apparent with increasing age. Mechanisms that control initiation and maintenance of locomotor behavior may become less responsive with age to periodic alternative of light and dark phases.