Light-dark masking of circadian temperature and activity rhythms in squirrel monkeys.

In addition to the synchronizing effects of light-dark cycles on the circadian timing system, light per se can have a direct masking effect on the rhythmic variables monitored as indicators of the circadian system. To evaluate the masking action of light on the rest-activity and body temperature rhythms of squirrel monkeys (Saimiri sciureus), animals were exposed to cycles of 2 h of light alternating with 2 h of darkness (LD 2:2). There was evidence of phase-dependent masking responses in both rhythms, with the light-induced elevation of activity levels and temperature greatest around the times of the circadian maxima of the respective rhythms. The temporal distribution of activity was more strongly affected by the masking action of the 2:2 cycle than was the temporal pattern of temperature variation. The increase in activity observed during lights on was evidently a major factor contributing to the corresponding increase in temperature.