Joint NASA-ESA-DARA Study. Part three: 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 affect human functioning and productivity. Therefore, a ground-based international study was performed that tested various psychological and physiological functions under the condition of two different levels of CO2. Thus, a specific experiment aimed at the influence of enhanced CO2-concentrations on the sleep-wake cycle and the circadian system.

Methods: Four volunteers were subjected to a CO2 concentration of 0.7% and 1.2%, respectively, for 24 consecutive days under each condition. Before they were exposed to these elevated levels, baseline measurements were conducted during 5 d under ambient CO2 concentrations (<0.1%). Circadian parameters were assessed by continuous recording of body temperature and by the determination of excretion rates of cortisol, 6-hydroximelatoninsulfate and catecholamines at 3-h intervals during wake. These measurements were conducted on 3 consecutive days 4 times during the two campaigns. Nondominant wrist activity and subjective fatigue was monitored throughout both study phases.

Results: Daily means of activity showed significant differences between the two study conditions (with lower values during the 1.2% condition), but these differences only occurred during day-time, whereas during sleep activity levels remained the same. Diurnal mean and circadian amplitude of the temperature rhythm was also significantly lower during the 1.2% condition.

Conclusions: The lower daily mean and circadian amplitudes of temperature during the higher CO2 level can be explained by lower physical activity that was monitored under this level, i.e., a masking effect. We hesitate to explain the lower activity (and hence temperature) as a response to the changed CO2 concentration. More probably, subjects became so acquainted with the procedures of the study protocol during the second study 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 circadian rhythm of healthy men.