The primate circadian timekeeping system in a hyperdynamic environment.

This study was designed to examine the possible effects of hyperdynamic fields (+G) on the circadian timekeeping system of the squirrel monkey. Eight monkeys were sequentially exposed to 1.0, 1.5, 2.0 and 1.0 G. During the two 1 G phases and the 2.0 G phase, the animals were exposed to constant light (LL) in addition to the 24-hr light-dark cycle (LD 12:12) present during the rest of the experiment. All animals were housed simultaneously on an 18 ft diameter centrifuge. The animals were allowed food and water ad lib, the intake of which was monitored electronically every 30 minutes. In all three acceleration fields, the animals demonstrated prominent circadian rhythms in both feeding and drinking. In the LD cycle, both rhythms were entrained with 24 hr periods. A phase delay occurred between the rhythms and the LD synchronizer in the +G environment. Moreover, when the animals were exposed to LL at 2.0 G, the mean free-running period was longer than when the animals were at 1 G either before or after the hyperdynamic exposure. In summary, this study demonstrates that the circadian timekeeping system of these primates is functional in the hyperdynamic environment, however, some of its components appear to be regulated at different homeostatic levels.