Two meals promote entrainment of rat food-anticipatory and rest-activity rhythms.

Ten female rats were fed early and late in the dark period of a 12-12 h light-dark cycle and then were fed at the same times in constant darkness. In both conditions rats were active prior to mealtimes and manifested no free-running components of activity. When the rats were placed in constant darkness and either were fed early and late in the inactive period, or had free access to food, six of the rats had rest-activity rhythms different from 24.0 h. Though a masking explanation could not be ruled out, two meals during the active period apparently entrained the rest-activity rhythms of these rats. The light-entrainable oscillator appears to integrate information from cycles of both illumination and food availability. Multiple sources of temporal information may promote more stable entrainment of the rest-activity rhythm than the light-dark cycle alone, especially in a burrow dwelling organism, like the rat, that can be exposed to inconsistent light-dark transitions.