Effects of square-wave and simulated natural light-dark cycles on hamster circadian rhythms

Abstract

Circadian rhythms of activity (Act) and body temperature (T(b)) were recorded from male Syrian hamsters under square-wave (LD(Sq)) and simulated natural (LD(SN), with dawn and dusk transitions) light-dark cycles. Light intensity and data sampling were under the synchronized control of a laboratory computer. Changes in reactive and predictive onsets and offsets for the circadian rhythms of Act and T(b) were examined in both lighting conditions. The reactive Act onset occurred 1.1 h earlier (P < 0.01) in LD(SN) than in LD(Sq) and had a longer α-period (1.7 h; P < 0.05). The reactive T(b) onset was 0.7 h earlier (P < 0.01) in LD(SN). In LD(SN), the predictive Act onset advanced by 0.3 h (P < 0.05), whereas the T(b) predictive onset remained the same as in LD(Sq). The phase angle difference between Act and T(b) predictive onsets decreased by 0.9 h (P < 0.05) in LD(SN), but the offsets of both measures remained unchanged. In this study, animals exhibited different circadian entrainment characteristics under LD(Sq) and LD(SN), suggesting that gradual and abrupt transitions between light and dark may provide different temporal cues.