Dim nocturnal illumination alters coupling of circadian pacemakers in Siberian hamsters, Phodopus sungorus


The circadian pacemaker of mammals comprises multiple oscillators that may adopt different phase relationships to determine properties of the coupled system. The effect of nocturnal illumination comparable to dim moonlight was assessed in male Siberian hamsters exposed to two re-entrainment paradigms believed to require changes in the phase relationship of underlying component oscillators. In experiment 1, hamsters were exposed to a 24-h light-dark-light-dark cycle previously shown to split circadian rhythms into two components such that activity is divided between the two daily dark periods. Hamsters exposed to dim illumination (<0.020 lx) during each scotophase were more likely to exhibit split rhythms compared to hamsters exposed to completely dark scotophases. In experiment 2, hamsters were transferred to winter photoperiods (10 h light, 14 h dark) from two different longer daylengths (14 h or 18 h light daily) in the presence or absence of dim nighttime lighting. Dim nocturnal illumination markedly accelerated adoption of the winter phenotype as reflected in the expansion of activity duration, gonadal regression and weight loss. The two experiments demonstrate substantial efficacy of light intensities generally viewed as below the threshold of circadian systems. Light may act on oscillator coupling through rod-dependent mechanisms.

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activity duration


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evening oscillator


estimated testis volume


light-dark-light-dark cycle


light emitting diode


morning oscillator


suprachiasmatic nuclei


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We are grateful to Antonio Mora and Tony Mora for excellent animal care, and to Magdalena Kendall and Mona Fallah-Tafti for assistance with analysis. This research was supported by NIH grants HD-36460 and NS-30235 and NSF grant IBN-0346391 and was conducted in compliance with all rules and regulations of the Animal Care and Use Committee, University of California, San Diego and the USDA, and followed recommendations in Guide for the Care and Use of Laboratory Animals.

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Correspondence to M. R. Gorman.

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Gorman, M.R., Elliott, J.A. Dim nocturnal illumination alters coupling of circadian pacemakers in Siberian hamsters, Phodopus sungorus . J Comp Physiol A 190, 631–639 (2004). https://doi.org/10.1007/s00359-004-0522-7

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  • Oscillator photoperiod coupling
  • Splitting entrainment