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Circadian rhythms of body temperature and motor activity were documented in young and old rats (four 8-week-old and five 22-month-old male Wistars, implanted with telemetric probes and housed in a chronobiological facility) under two different photoperiod conditions. The animals were maintained in a light:dark (LD) cycle of 12 h each (LD 12:12) for 4 weeks and then exposed to a LD 6:18 cycle for 7 weeks to assess the effect of age on the desynchronization of the temporal structure of the rhythms. In old rats under LD 12:12, the power of the 24-h component and the circadian amplitude of body temperature and motor activity were markedly lower than in the young and both rhythms were phase-advanced. After the shift to LD 6:18, the circadian rhythmicity was maintained for both variables and the same phase delay (+5+/-1 h) was observed in both age groups, as was a gradual expansion of the patterns of both functions with the longer night. The photoperiod reduction (6 weeks under LD 6:18) did not modify the power of the 24-h component of body temperature and motor activity in old rats. In young rats, however, the power and amplitude of the 24-h component of motor activity rhythm fell to the levels of those in old rats, while the power of the 24-h component of body temperature rhythm and the amplitude did not change. Our data show that the circadian rhythm of motor activity, but not of body temperature, responds age dependently to a photoperiod reduction.

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