Seasonal acclimation is associated with various behavioral and physiological changes, allowing animals to cope with different climatic conditions and food availability. These changes are timed by the annual photoperiodic cycle. In some species, it entrains an endogenous circannual rhythm to a period of 1 year. In other species, the photoperiodic cycle directly drives the seasonal changes in a given function (or at least one of the transitional phases of this function). Whatever the underlying mechanism is, it enables the organism to prepare for rather than merely react to cyclic events in the environment. Most organisms use circadian oscillators to measure length of day and, in mammals, the suprachiasmatic nuclei (SCN) play a crucial role. These structures receive photic stimuli via the retino-hypothalamic tract and encode information about photoperiod via modification of clock gene expression and reorganization of neuronal networks. The resulting signal, in turn, regulates pineal melatonin synthesis and release. It is mainly the duration of the melatonin signal that provides the information about photoperiod and the direction (lengthening or shortening) of its change. This hormone is secreted at night and transported to various target sites where the signal is “decoded”. The duration of secretion is inversely related to day length. In Djungarian hamsters (Phodopus sungorus) of our breeding stock, a certain number of animals show a delayed activity onset (DAO) which is caused by a diminished ability to photic synchronization. Other hamsters show arrhythmic activity patterns (AR) because the SCN as the main circadian oscillator does not generate a circadian signal. Since, a functional circadian system is a prerequisite for photoperiodic time measurement the question arose, what consequences these deteriorations may have for seasonal adaptation. When animals of the different rhythmic phenotypes were transferred to short day (SD) conditions (L:D = 8:16 h), none of the DAO and AR hamsters displayed any SD trait. Only wild type (WT) hamsters did respond properly. Their activity time expanded, body mass and testes size decreased, and fur coloration changed from summer to winter pelage. Only a small number of hamsters did not respond, a phenomenon that has been described before. Possibly, this is of adaptive significance, as these animals may reproduce late in the year if the environmental conditions are suitable. Non-responding hamster, we also found under natural lighting and temperature conditions. Moreover, these animals did not display daily torpor, a state that was regularly observed in responders. This way, hamsters do save energy under conditions of low outdoor temperature. The times of torpor onset and offset are under circadian control. When DAO hamsters were kept in constant darkness (DD), they exhibited typical short-day traits like lengthening of activity time, reduction of body mass and testes size, and change of fur coloration. Since, under these conditions, non-parametric light effects inducing phase responses are absent, activity-onset and activity-offset (~evening and morning oscillators) may change according to the oscillators’ intrinsic period lengths. Both are >24 h, but the period of the evening oscillator is shorter than that of the morning oscillator, and this is the reason for the expansion of activity time in DD. The decompression may cause a longer melatonin signal and, subsequently, the short-day traits that were obtained. By contrast, AR animals did not display any SD response even after 14 weeks in DD. Obviously they are incapable of measuring photoperiodic time due to a complete disruption of circadian rhythmicity. Their SCN do not generate a circadian rhythm. The results show physiological mechanisms necessary for seasonal adaptation are working in DAO hamsters and that it is the inadequate interaction of the LD cycle with the SCN that prevents the photoperiodic reaction. The signal released by the SCN and sent to the pineal is not consistent with the photoperiod. As a consequence of this disruption in the circadian system, the pattern of melatonin synthesis does not reflect the length of night appropriately. We have shown that the circadian melatonin rhythm in DAO hamsters kept under long day conditions correlates with their delayed activity-onset; a similar pattern might be expected when these hamsters are kept in SD. In summary, one may conclude that only animals with a functional circadian system, the WT hamsters, can respond properly to changes of the photoperiod and, thus, adapt to seasonal variations in the environment. Hamsters of the other two phenotypes not only lack the ability to synchronize with the 24-h day, but also the capability of photoperiodic time measurement.