Abstract: In Siberian hamsters (Phodopus sungorus), long day lengths maintain the summer phenotype, while short day induce the winter phenotype. After several months of continued short days, however, hamsters become refractory to short days, and the spring phenotype spontaneously re-emerges. Many weeks of long days are then thought to be necessary to break refractoriness. Most photoperiodism studies have focused on responses to abrupt changes between static day lengths, but these studies lack ecological relevance and may obscure normal responses to incrementally changing photoperiods. In the first experiments, simulated natural photoperiods (SNPs) synchronized the seasonal cycles of asynchronously born male and female hamsters. For all hamsters, the transition from summer-like rapid maturation to a winter strategy of delayed puberty occurred at long day lengths which induce early puberty when presented as static photoperiods. The effects of prenatal day length on postnatal development were smaller than expected in males, and absent in females. SNPs revealed sex differences in responsiveness to natural photoperiod variations. After the winter solstice, 19 weeks of increasing day lengths broke refractoriness in the majority of male hamsters. To determine if absolutely long days are necessary to break refractoriness, some hamsters were pinealectomized at the spring equinox to remove endogenous encoding of photoperiod by melatonin. Ten weeks later, melatonin infusions caused testicular regression in these hamsters, indicating that short but increasing day lengths to a maximum of only 12.5 h were sufficient to break refractoriness many weeks later. Photorefractoriness has been attributed to the development of refractoriness of the neuroendocrine system, and specifically of the suprachiasmatic nucleus (SCN), to long duration melatonin signals. I therefore tested whether this renders the circadian system of Siberian hamsters unresponsive to melatonin's chronobiotic effects. The magnitude of circadian phase shifts to melatonin injections did not differ between photosensitive and photorefractory hamsters. A subset of SCN cells of photorefractory hamsters remains responsive to melatonin's circadian effects, even as these or other SCN cells no longer transduce the effects of melatonin on seasonal rhythms.