Twilight transitions promote circadian entrainment to lengthening
light-dark cycles.
Boulos, Z., M. Macchi, and M. Terman.
Institute for Circadian Physiology, One Alewife Center, Cambridge,
MA 02140
APStracts 3:0239R, 1996.
The upper limits of entrainment of the circadian activity rhythm were
compared in hamsters initially exposed to daily light-dark (LD)
cycles with either abrupt (LD-rectangular) or simulated twilight
transitions (LD-twilight). Daytime illuminance (10 lux) and the total
amount of light emitted per day were the same under the two LD
cycles. Half the animals in each condition had access to dark nest
boxes. The period of the LD cycles was then increased from 24 h to 26
h, by 5 min per day. All animals in LD-twilight remained entrained to
the lengthening cycle, while 60% of those in LD-rectangular began to
free-run well before the period of the cycle reached 26 h. These
effects were independent of nest box availability. The lengthening LD
cycles exerted clear after-effects on the period of the rhythms in
constant darkness, the magnitude of which was related to the efficacy
of prior entrainment. The results indicate that twilight transitions
raise the upper limit of entrainment to LD cycles, suggesting that
their inclusion increases the strength of the LD zeitgeber.
Received 8 March 1996; accepted in final form 31 May 1996.
APS Manuscript Number R142-6.
Article publication pending Am. J. Physiol. (Regulatory Integrative
Comp. Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 4 July 96