Midnight Siesta and Circadian Rhythms of Related Metabolic and Behavioral Variables in Aging Mice

Introduction: Circadian rhythms are genetically programmed traits coordinating behavior and metabolic responses with daily repeating environmental challenges, primarily with the solar day. Aging and circadian rhythms are interrelated: aging can modify the circadian rhythms, whereas age-changes of the rhythms contribute to the declined adaptability in aging. Material and Methods: Circadian rhythms of non-invasive metabolic variables (sleep/wake, locomotor activity, production of CO2, consumption of O2, food and water) were studied in young, middle aged and old males of two mouse strains (FVBcg and CBA) at the naturally alternating 12:12 h dark/light regime (around the March equinox). Results: All studied rhythms demonstrated a highly significant circadian component (P<10-6). The impact of the age-associated factors or combined action of the age and circadian factors was less pronounced. Aging was associated with declined amplitude of circadian rhythms of many but not all studied indices. The number of wake old mice at the daytime hours was increased, supporting the concept of age-associated insomnia. Nighttime activity of CBA mice was characterized by two maximums at the after-dusk and pre-down hours divided by a period of siesta (resting/sleeping) at the midnight. As in humans, the mouse siesta was more common in the young and old animals than in the middle aged group. Conclusions: Aging is associated with alterations of the metabolic and behavioral circadian rhythms. Data support the concept of insomnia in the advanced ages but disagree with the contention that the highest activity of mice occurs at the midnight. It rather indicates that siesta could be dominating at those hours, especially among the young and old mice. Further research is warranted to clarify the role of circadian rhythms and siesta in aging and longevity.