Circadian rhythms in SAMP8: A longitudinal study of the effects of age and experience

Age-related effects on circadian rhythms include reductions of rhythm amplitude, alterations in re-entrainment, and increased fragmentation. Currently, the pattern of these changes across an individuals' lifespan is unknown. The present study used a cross-sequential experimental design to determine the pattern of circadian rhythm changes, identify predictors of later circadian rhythm disruption, and assess the effect of prior run-wheel experience on circadian rhythms. Run-wheel activity was assessed in senescence-accelerated mice (SAMP8) at 2, 7, and 12 months of age. Age-related changes included decrease of run-wheel activity, decrease in circadian rhythm amplitude, increase in proportion of light activity, and increase in split activity rhythms. Proportion of light activity at 2 months was a good predictor of circadian rhythm disruption at 7 months. Run-wheel experience increased overall activity and decreased proportion of light activity, but did not alter rhythm amplitude or period. These results demonstrate that aging produces several patterns of circadian rhythm changes, describe predictive measures of future rhythm disruptions, and suggest an intervention to reduce circadian rhythm disruptions.