Primate circadian rhythms during spaceflight: results from Cosmos 2044 and 2229.

The circadian timing system (CTS) coordinates an animal's physiology and behavior both internally and with the 24-h day. Previous studies have suggested that the CTS is sensitive to changes in gravity. To examine this question, the expression of the CTS in four juvenile male rhesus macaques (Macaca mulatta) were studied in space. These animals were flown on the Cosmos 2044 and 2229 missions. Activity, heart rate, and axillary and brain (Cosmos 2229) temperatures were recorded. In both flights, the subjects exhibited delays in the phasing of their temperature rhythms and a decrease in mean heart rate compared with ground control studies. These data are in support of other studies that demonstrate that the CTS is sensitive to changes in the gravitational environment. Furthermore, the data also support the concept of a multioscillator organization of the primate CTS due to the differential responses of the rhythms measured.