A recent study investigating the effects of long-duration spaceflight on CBT at rest and during exercise are clearly lacking. We here show that during exercise CBT rises higher and faster in space than on Earth. Moreover, we observed for the first time a sustained increased astronauts’ CBT also under resting conditions.

This increase of about 1 °C developed gradually over 2.5 months and was associated with augmented concentrations of interleukin-1 receptor antagonist. Since even minor increases in CBT can impair physical and cognitive performance, both findings have a considerable impact on astronauts’ health and well-being during future long-term spaceflights. Moreover, our findings also pinpoint crucial physiological challenges for spacefaring civilizations, and raise questions about the assumption of a thermoregulatory set point in humans

Our evolutionary ability to adapt to climate changes on Earth. Various studies dealt with the impact of physical activity, clothing, and environmental factors on CBT regulation under terrestrial conditions. Elevated CBT in space has been earlier observed by Gundel, using 24-hours CBT recordings, however to much lower extent (0.06–0.16 °C). In this case the higher resting CBT remained elevated throughout the stay on the ISS and finally returned to baseline 10 days after landing.

However, these studies were short-term missions, spanning a maximum of 8–16 days of spaceflight. Our data suggest that the shift in CBT seems to be a slow, gradual phenomenon that reaches its peak only after months of spaceflight. In addition, we also observed an increase of maximal CBT after short bouts of exercise, in some cases exceeding 40 °C. Remarkably, these increases occurred despite workloads were lower inflight compared to the preflight exercise protocol.

Moreover, the rate of the increase in CBT during exercise, i.e. the slope of CBT, was significantly steeper during spaceflight. These findings are very much in line with earlier studies, stating that after long-term spaceflight the sensitivity of the heat-loss responses is reduced, resulting in a faster rise in CBT. Furthermore, our data indicate that the impairments in thermoregulation are still prevalent after return to Earth, and recovery occurs only very gradually. In our opinion, the diminished convective and evaporative heat loss in space called for an increase in skin perfusion to enhance radiative heat loss.

It might be an attempt of the autonomic nervous system to increase heat loss through radiation even under resting conditions in space. We found that CBT rises higher and faster during physical exercise in space than on ground, and resting CBT is elevated in long-duration spaceflight. We concluded that, within the limits of a spaceflight experiment, these increases might be related to persistent low-grade pro-inflammatory responses to weightlessness, strenuous exercise protocols, radiation, psychological stress-induced hyperthermia.