The free-run period is not the best Zeitgeber period for the house mouse

Two light cycles were presented simultaneously to house mice. One cycle had a period of 23.7 hr, the free-run period of the activity cycle in constant darkness, and the other a period of 24.0 hr. The activity cycles of all 31 subjects synchronized temporarily with the 24.0 hr light cycle. In contrast, only 10 mice also showed temporary synchronization with the 23.7 hr light cycle and only 2 of these spent more time synchronized with the 23.7 than the 24.0 hr light cycle. The results suggest that a relaxation oscillator is not an adequate model of the mouse circadian activity cycle. The results may have practical significance for the design of human temporal environments.