Restricted feeding: a nonphotic zeitgeber in the rabbit.

The free-running circadian rhythms of five behavioral functions of the rabbit were masked by unsignalled restricted food access (RF). The rhythms were reorganized immediately, a large part of events being assembled around the end of food availability. In addition to masking a slower process of entrainment was running: a component of anticipatory activity (AA) was established 1-3 h before food access. AA consolidated in continuation of the camouflaged free-running rhythm. The time of AA establishment correlated significantly with the phase angle difference (PAD) between free-running rhythm and RF: it decreased with decreasing positive PAD. With the consolidation of AA, the rhythms had attained a stable phase relation to RF. At the termination of RF, the circadian rhythms started to free-run again, the phase being dependent from that of the preceding RF schedule. The period length of the RF zeitgeber (TRF) had some impact on tau of the circadian rhythm. This aftereffect was most evident after termination of TRF less than 24 h: the free-running rhythm continued for up to 49 days with a tau less than 24 h and turned longer thereafter. The results demonstrate the entrainment of circadian rhythms of the rabbit by RF in addition to masking.