The primate circadian timekeeping system in a hyperdynamic environment

The effect of hyperdynamic field (12 d at 1.5 G and 35 d at 2.0 G phases, preceded and followed by 1.0 G phases of 15 and 18 days, respectively) on the circadian rhythm in the feeding and drinking of the squirrel monkey was studied. Two lighting regimens were employed: (1) the 24-hr light-dark cycle (LD 12:12) at all G phases and (2) constant light (LL) during the two 1.0 G phases and a 2.0 G phase. In the LD regimen, both feeding and drinking rhythms were entrained with the 24-hr periods at all G levels, but a phase delay occurred in high G environment. In the LL regimen, the rhythm persisted with a free-running period greater than 24 hours and an increase at 2.0 G, compared to the 1.0 G phases. Thus, although the circadian rhythm is functional at high G, some of its components appear to be regulated at different homeostatic levels.