Regulatory, Integrative and Comparative Physiology

Constant darkness restores entrainment to phase-delayed Siberian hamsters

Norman F. Ruby, Nirav Joshi, H. Craig Heller


Over 90% of Siberian hamsters (Phodopus sungorus) fail to reentrain to a 5-h phase delay of a 16:8-h photocycle. Because constant darkness (DD) restores rhythms disrupted by constant light, we tested whether DD could also restore entrainment. DD began 0, 5, or 14 days after a 5-h phase delay, and the light-dark cycle was reinstated 14 days later. All hamsters exposed to DD on day 0 reentrained, whereas 42% reentrained irrespective of whether DD began 5 or 14 days later. For these latter two groups, tau (τ) and alpha (α) in DD predicted reentrainment; animals that reentrained had a mean τ and α of 24.1 and 8.9 h, respectively, whereas those that failed to reentrain maintained a mean τ and α of 25.0 and of 7.1 h, respectively. Restoration of entrainment by DD is somewhat paradoxical because it suggests that reentrainment to the photocycle was prevented by continued exposure to that same photocycle. The dichotomy of circadian responses to DD suggests “entrainment” phenotypes that are similar to those of photoperiodic responders and nonresponders.

  • circadian
  • phase shift
  • light-dark cycle
  • activity rhythm


  • This work was supported by National Institutes of Health Grant MH-60385.

  • Address for reprint requests and other correspondence: N. F. Ruby, Dept. of Biological Sciences, 371 Serra Mall, Stanford Univ., Stanford, CA 94305-5020 (E-mail:ruby{at}

  • The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

  • First published August 22, 2002;10.1152/ajpregu.00362.2002

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