Normally, internal clocks are dependent on light. Therefore, if a free-running animal is exposed to constant periods of light and dark, their circadian rhythm can be shifted. For example, a free-running hamster exposed to set periods of light and dark can adjust the hamster's cycle to do its wheel running in the dark period instead of the light period. The change in the circadian rhythm is called a phase shift and the process of synchronizing the rhythm to the environmental stimulus is called entrainment. Zeitgebers are any cues that animals use to synchronize their rhythm with the environment, and it is usually the light-dark cycle. Usually, the zeitgeber entrains circadian rhythms, which causes a phase shift.