Djungarian hamsters (Phodopus sungorus) exhibit pronounced winter acclimatization with changes in body mass, gonads, fur, and thermogenic capacity induced by decreasing daylength. To determine whether the annual activity pattern reflects the crucial role of the photoperiod for the hamsters' seasonality, animals with and without access to a running wheel (RW) were exposed to natural lighting conditions (approximately 52 degrees N) and ambient temperatures. Registration of locomotion in hamsters with a RW revealed a clear activity pattern closely related to dusk and dawn throughout the year. In contrast, animals without RW access showed a less stable phase relationship between the activity and the day-night cycle in autumn and winter. During these seasons, the activity phase either exceeded the dark phase or even became indistinguishable from the rest phase. This correlated not only with increased locomotion during the light phase but also over the whole 24 h period, especially in autumn. In RW hamsters, a similar but attenuated trend was found that possibly reflects foraging due to increased food hoarding before winter. The more stable correlation between activity time and night length in RW hamsters might be explained by a suppressing effect of light on wheel-running behavior (negative masking) and/or a stabilizing effect of running exercise on rhythmicity. In a further experiment, the phase-reference points lights-off and lights-on within artificial light-dark (LD) cycles were compared to sunset and sunrise in an intermediate ratio of light and dark and in long days. With respect to the defined phase-reference points of the zeitgeber, the phase relation between activity and the LD cycle was similar in natural and corresponding artificial lighting conditions, and dependent on the LD ratio.