JOINT NASA-ESA-DARA STUDY - PART 3 - CIRCADIAN-RHYTHMS AND ACTIVITY-REST CYCLE UNDER DIFFERENT CO2 CONCENTRATIONS

Background: During manned space missions, the actual concentration of carbon dioxide (CO2) is frequently elevated to a degree that may affec t human functioning and productivity. Therefore, a ground-based intern ational study was performed that tested various psychological and phys iological functions under the condition of two different levels of CO2 . Thus, a specific experiment aimed at the influence of enhanced CO2-c oncentrations on the sleep-wake cycle and the circadian system. Method s: Four volunteers were subjected to a CO2 concentration of 0.7% and 1 .2%, respectively, for 24 consecutive days under each condition. Befor e they were exposed to these elevated levels, baseline measurements we re conducted during 5 d under ambient CO2 concentrations (<0.1%). Circ adian parameters were assessed by continuous recording of body tempera ture and by the determination of excretion rates of cortisol, 6-hydrox imelatoninsulfate and catecholamines at 3-h intervals during wake. The se measurements were conducted on 3 consecutive days 4 times during th e two campaigns. Nondominant wrist activity and subjective fatigue was monitored throughout both study phases. Results: Daily means of activ ity showed significant differences between the two study conditions (w ith lower values during the 1.2% condition), but these differences onl y occurred during day-time, whereas during sleep activity levels remai ned the same. Diurnal mean and circadian amplitude of the temperature rhythm was also significantly lower during the 1.2% condition. Conclus ions: The lower daily mean and circadian amplitudes of temperature dur ing the higher CO2 level can be explained by lower physical activity t hat was monitored under this level, i.e., a masking effect. We hesitat e to explain the lower activity land hence temperature) as a response to the changed CO2 concentration. More probably, subjects became so ac quainted with the procedures of the study protocol during the second s tudy phase (1.2% CO2) that they did not need similar effort to conduct the tasks as during the first phase. The activity level during sleep which was the same during the two phases may support this conclusions. Thus, we conclude that CO2 levels up to 1.2% do not impair the circad ian rhythm of healthy men.