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

Circadian rhythms of activity (Act) and body temperature (T-b) were recorde d from male Syrian hamsters under square-wave (LDSq) and simulated natural (LDSN, with dawn and dusk transitions) Light-dark cycles. Light intensity a nd data sampling were under the synchronized control of a laboratory comput er. Changes in reactive and predictive onsets and offsets for the circadian rhythms of Act and Tb were examined in both lighting conditions. The react ive Act onset occurred 1.1 h earlier (P < 0.01) in LDSN than in LDSq and ha d a longer alpha-period (1.7 h; P < 0.05). The reactive T-b onset was 0.7 h earlier (P < 0.01) in LDSN. In LDSN, the predictive Act onset advanced by 0.3 h (P < 0.05), whereas the T-b predictive onset remained the same as in LDSq. The phase angle difference between Act and Tb predictive onsets decre ased by 0.9 h (P < 0.05) in LDSN, but the offsets of both measures remained unchanged. In this study, animals exhibited different circadian entrainmen t characteristics under LDSq and LDSN, suggesting that gradual and abrupt t ransitions between light and dark may provide different temporal cues.