Vol. 276, Issue 4, R1195-R1202, April 1999
Effects of square-wave and simulated natural light-dark cycles
on hamster circadian rhythms
I-Hsiung
Tang,
Dean M.
Murakami, and
Charles A.
Fuller
Section of Neurobiology, Physiology, and Behavior, University of
California, Davis, California 95616-8519
Circadian
rhythms of activity (Act) and body temperature
(Tb) were recorded from male
Syrian hamsters under square-wave
(LDSq) and simulated natural
(LDSN, 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 Tb were examined in both
lighting conditions. The reactive Act onset occurred 1.1 h earlier
(P < 0.01) in
LDSN than in
LDSq and had a longer 
-period
(1.7 h; P < 0.05). The reactive
Tb 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 Tb predictive onset
remained the same as in LDSq. The
phase angle difference between Act and Tb predictive onsets decreased by
0.9 h (P < 0.05) in
LDSN, but the offsets of both
measures remained unchanged. In this study, animals exhibited different
circadian entrainment characteristics under
LDSq and
LDSN, suggesting that gradual and
abrupt transitions between light and dark may provide different
temporal cues.
light-emitting diode; masking; activity; body temperature; dawn; dusk
