Humans' core body temperature (CBT) is strictly controlled within a narrow range. Various studies dealt with the impact of physical activity, clothing, and environmental factors on CBT regulation under terrestrial conditions. However, the effects of weightlessness on human thermoregulation are not well understood. Specifically, studies, 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, a key anti-inflammatory protein. 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, and our evolutionary ability to adapt to climate changes on Earth.

Biomedical Research and Environmental Sciences Division NASA Johnson Space Center Houston TX 77058 USA

Charité Medizinische Klinik Charité Universitätsmedizin Berlin Kardiologie Augustenburger Platz 1 Berlin 13353 Germany

Charité Universitätsmedizin Berlin corporate member of Freie Universität Berlin Humboldt Universität zu Berlin and Berlin Institute of Health Institute of Physiology Center for Space Medicine and Extreme Environments CharitéCrossOver Charitéplatz 1 Berlin 10117 Germany

Department of Anaesthesiology Hospital of the University of Munich Marchioninistrasse 15 München 81377 Germany

Department of Biomedical Sciences for Health Università degli Studi di Milano via Luigi Mangiagalli 31 20133 Milan Italy

Department of Health and Kinesiology Lamar University Beaumont TX 77710 USA

Division of Sleep and Chronobiology Department of Psychiatry Perelman School of Medicine at the University of Pennsylvania 1019 Blockley Hall 423 Guardian Drive Philadelphia PA 19104 6021 USA

DLR Institut für Luft und Raumfahrtmedizin Abteilung Strahlenbiologie Linder Höhe Köln 51147 Germany

Drägerwerk AG and Co KGaA Moislinger Allee 53 55 Lubeck 23558 Germany

German Air Force Centre of Aerospace Medicine Aviation Physiology Training Centre Aviation Physiology Diagnostics and Science Steinborner Str 43 01936 Königsbrück Germany

Key Laboratory for Space Bioscience and Biotechnology Institute of Special Environnments Biophysics School of Life Sciences Northwestern Polytechnical University Xi'an 710072 China

Nuclear Physics Institute of the Czech Academy of Sciences Department of Radiation Dosimetry Na Truhlářce 39 64 Praha 8 180 00 Czech Republic

Preventive Medicine and Community Health University of Texas Medical Branch 301 University Boulevard Galveston TX 77555 USA

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