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Scotopic Illumination Enhances Entrainment of Circadian Rhythms to Lengthening Light:Dark Cycles

Department of Psychology, 9500 Gilman Drive, University of California, San Diego, La Jolla, CA 92093-0109;mgorman{at}ucsd.edu

Magdalena Kendall

Department of Psychology, University of California, San Diego, La Jolla, CA 92093

Jeffrey A. Elliott

Department of Psychiatry, University of California, San Diego, La Jolla, CA 92093

Endogenously generated circadian rhythms are synchronized with the environment through phase-resetting actions of light. Starlight and dim moonlight are of insufficient intensity to reset the phase of the clock directly, but recent studies have indicated that dim nocturnal illumination may otherwise substantially alter entrainment to bright lighting regimes. In this article, the authors demonstrate that, compared to total darkness, dim illumination at night (< 0.010 lux) alters the entrainment of male Syrian hamsters to bright-light T cycles, gradually increasing in cycle length (T) from 24 h to 30 h. Only 1 of 18 hamsters exposed to complete darkness at night entrained to cycles of T > 26 h. In the presence of dim nocturnal illumination, however, a majority of hamsters entrained to Ts of 28 h or longer. The presence or absence of a running wheel had only minor effects on entrainment to lengthening light cycles. The results further establish the potent effects of scotopic illumination on circadian entrainment and suggest that naturalistic ambient lighting at night may enhance the plasticity of the circadian pacemaker.

Key Words: Tcycles • moonlight • coupling • circadian entrainment • illuminance • irradiance • wheel running • hamster

Journal of Biological Rhythms, Vol. 20, No. 1, 38-48 (2005)
DOI: 10.1177/0748730404271573


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