Entrainment and masking of circadian drinking rhythms in primates: influence of light intensity.

The entrained drinking rhythms of squirrel monkeys were studied during exposure to 24 hr illumination cycles of three different intensities (60:0, 66:6, 76:16 lux). Increasing the intensity of ambient illumination significantly delayed the offset of drinking but had no effect on either the onset or the total amount of daily drinking behavior. Comparison of drinking behavior under an alternating schedule of LD 66:6 lux and constant light of 6 lux indicated that the twice daily light transitions consistently altered the temporal distribution of drinking behavior. The daily timing of squirrel monkey drinking behavior thus, depends not only o the mechanisms of circadian entrainment to the LD cycle, but also on the ability of the LD cycle to directly influence, or "mask" behavior.