Regulation of testis function in golden hamsters: a circadian clock measures photoperiodic time.

The photoperiodic testicular response of adult golden hamsters was examined by the use of a 6-hour light period coupled with dark periods of 18, 30, 42, and 54 hours. Cycle lengths of 24 and 48 hours resulted in testicular regression, whereas testicular weight was maintained by cycle lengths of 36 and 60 hours. Our data demonstrate a circadian rhythm of sensitivity to the effects of light on the photoperiodic testicular response of the hamster. The position of light relative to the circadian system (as measured by the locomotor rhythm) is critical in the response.