Light masking of circadian rhythms of heat production, heat loss, and bodytemperature in squirrel monkeys

Whole body heat production (HP) and heat loss (HL) were examined to determi ne their relative contributions to light masking of the circadian rhythm in body temperature (T-b) Squirrel monkey metabolism (n = 6) was monitored by both indirect and direct calorimetry, with telemetered measurement of body temperature and activity. Feeding was also measured. Responses to an entra ining light-dark (LD) cycle (LD 12:12) and a masking LD cycle (LD 2:2) were compared. HP and HL contributed to both the daily rhythm and the masking c hanges in T-b All variables showed phase-dependent masking responses. Maski ng transients at L or D transitions were generally greater during subjectiv e day; however, L masking resulted in sustained elevation of T-b, HP, and H L during subjective night. Parallel, apparently compensatory, changes of HZ and HP suggest action by both the circadian timing system and Light maskin g on T-b set point. Furthermore, transient HL increases during subjective n ight suggest that gain change may supplement set point regulation of T-b.