Change in duration of the nighttime melatonin peak may be a signal driving photoperiodic responses in the Djungarian hamster (Phodopus sungorus).

A regime of 14 h of light and 10 h of darkness (LD 14:10) induced regression of testes and accessory glands in adult Djungarian hamsters maintained originally in LD 16:8, but recrudescence of these organs in hamsters maintained originally in LD 8:16. Duration of the period of elevated nighttime melatonin levels in LD 14:10 was the same irrespective of whether hamsters had come from LD 16:8 or from LD 8:16. It is suggested that a change in duration of the nighttime melatonin peak and its direction rather than absolute duration of the melatonin peak may be a hormonal signal driving photoperiodic responses in adult Djungarian hamsters.