Sex difference in circadian period of body temperature in Clock mutant mice with Jcl/ICR background.

The period of circadian rhythm remains remarkably stable in both sexes under conditions of constant darkness. Several studies have indicated that gonadal hormones such as estradiol shorten the period of circadian rhythms. We observed that the free-running period of core body temperature is longer in male homozygous Clock mutant mice than in females, although this difference is not found in wild mice. Furthermore, a gonadectomy had no effect on the period of core body temperature in either sex. This suggests the existence of a sex-related difference of the mammalian circadian system, and that it is independent of peripheral gonadal regulations.