Joint NASA-ESA-DARA Study. Part three: circadian rhythms and activity-rest cycle under different CO2 concentrations

Area of Science:

• Space Medicine
• Human Physiology
• Environmental Health

Background:

• Elevated carbon dioxide (CO2) concentrations in manned space missions can impact human functioning.
• A ground-based study investigated the effects of increased CO2 on psychological and physiological functions.
• The experiment specifically focused on the influence of enhanced CO2 on the sleep-wake cycle and circadian system.

Purpose of the Study:

• To assess the impact of elevated carbon dioxide (CO2) concentrations on human circadian rhythms.
• To determine if CO2 levels experienced during space missions affect sleep-wake cycles and physiological parameters.
• To evaluate the relationship between CO2 exposure and human performance and well-being.

Main Methods:

• Four volunteers were exposed to 0.7% and 1.2% CO2 concentrations for 24 days each, with a baseline of ambient CO2 (<0.1%).
• Circadian parameters were measured via continuous body temperature recording and analysis of cortisol, 6-hydroximelatoninsulfate, and catecholamine excretion rates.
• Wrist activity and subjective fatigue were monitored throughout the study phases.

Main Results:

• Daily activity means were lower during the 1.2% CO2 condition, primarily during daytime, with no change during sleep.
• The diurnal mean and circadian amplitude of the temperature rhythm were significantly reduced at the 1.2% CO2 level.
• Lower activity and temperature at higher CO2 levels were attributed to a masking effect and potential habituation to study procedures, not a direct CO2 response.

Conclusions:

• CO2 levels up to 1.2% did not demonstrate impairment of the circadian rhythm in healthy individuals.
• Observed reductions in activity and temperature were likely due to masking effects and habituation, rather than direct CO2 impact.
• The study suggests that current CO2 levels in space missions may not negatively affect the fundamental circadian rhythm of healthy astronauts.