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

See more in PubMed

Hancock PA, Ross JM, Szalma JL. A meta-analysis of performance response under thermal stressors. Hum Factors. 2007;49:851–877. doi: 10.1518/001872007X230226. PubMed DOI

Taylor L, Watkins SL, Marshall H, Dascombe BJ, Foster J. The Impact of Different Environmental Conditions on Cognitive Function: A Focused Review. Front Physiol. 2015;6:372. PubMed PMC

Bouchama A, Knochel JP. Heat stroke. N Engl J Med. 2002;346:1978–1988. doi: 10.1056/NEJMra011089. PubMed DOI

Kjellstrom T, et al. Heat, Human Performance, and Occupational Health: A Key Issue for the Assessment of Global Climate Change Impacts. Annu Rev Public Health. 2016;37:97–112. doi: 10.1146/annurev-publhealth-032315-021740. PubMed DOI

Jacklitsch, B. et al. NIOSH criteria for a recommended standard: occupational exposure to heat and hot environments. U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication 2016–106.

Clark, R. P. & Edholm, O. G. Man and his thermal environment (Arnold London, 1985).

Mekjavic IB, Eiken O. Contribution of thermal and nonthermal factors to the regulation of body temperature in humans. Journal of applied physiology. 2006;100:2065–2072. doi: 10.1152/japplphysiol.01118.2005. PubMed DOI

Flouris AD, Schlader ZJ. Human behavioral thermoregulation during exercise in the heat. Scandinavian journal of medicine & science in sports. 2015;25:52–64. doi: 10.1111/sms.12349. PubMed DOI

Kenny, G. P. & Jay, O. Thermometry, calorimetry, and mean body temperature during heat stress. Comprehensive Physiology (2013). PubMed

Fortney SM, et al. Body temperature and thermoregulation during submaximal exercise after 115-day spaceflight. Aviation, space, and environmental medicine. 1998;69:137–141. PubMed

Polyakov VV, Lacota NG, Gundel A. Human thermohomeostasis onboard “Mir” and in simulated microgravity studies. Acta astronautica. 2001;49:137–143. doi: 10.1016/S0094-5765(01)00091-1. PubMed DOI

Vessel, E. A. & Russo, S. Effects of Reduced Sensory Stimulation and Assessment of Countermeasures for Sensory Stimulation Augmentation. A Report for NASA Behavioral Health and Performance Research: Sensory Stimulation Augmentation Tools for Long Duration Spaceflight (NASA/TM-2015-218576). NASA Center for AeroSpace Information (2015).

Crucian BE, et al. Plasma cytokine concentrations indicate that in vivo hormonal regulation of immunity is altered during long-duration spaceflight. J Interferon Cytokine Res. 2014;34:778–786. doi: 10.1089/jir.2013.0129. PubMed DOI PMC

Luheshi GN, Gardner JD, Rushforth DA, Loudon AS, Rothwell NJ. Leptin actions on food intake and body temperature are mediated by IL-1. Proc Natl Acad Sci USA. 1999;96:7047–7052. doi: 10.1073/pnas.96.12.7047. PubMed DOI PMC

Dijk DJ, et al. Sleep, performance, circadian rhythms, and light-dark cycles during two space shuttle flights. Am J Physiol Regul Integr Comp Physiol. 2001;281:R1647–64. PubMed

Gundel A, Nalishiti V, Reucher E, Vejvoda M, Zulley J. Sleep and circadian rhythm during a short space mission. Clin Investig. 1993;71:718–724. doi: 10.1007/BF00209726. PubMed DOI

Gundel A, Polyakov VV, Zulley J. The alteration of human sleep and circadian rhythms during spaceflight. J Sleep Res. 1997;6:1–8. doi: 10.1046/j.1365-2869.1997.00028.x. PubMed DOI

Crandall CG, Johnson JM, Convertino VA, Raven PB, Engelke KA. Altered thermoregulatory responses after 15 days of head-down tilt. J Appl Physiol (1985) 1994;77:1863–1867. PubMed

Stone JG, et al. Do standard monitoring sites reflect true brain temperature when profound hypothermia is rapidly induced and reversed. Anesthesiology. 1995;82:344–351. doi: 10.1097/00000542-199502000-00004. PubMed DOI

Gunga H-C, Sandsund M, Reinertsen RE, Sattler F, Koch J. A non-invasive device to continuously determine heat strain in humans. Journal of Thermal Biology. 2008;33:297–307. doi: 10.1016/j.jtherbio.2008.03.004. DOI

Gunga HC, et al. The Double Sensor-A non-invasive device to continuously monitor core temperature in humans on earth and in space. Respir Physiol Neurobiol. 2009;169(Suppl 1):S63–8. doi: 10.1016/j.resp.2009.04.005. PubMed DOI

Mendt S, et al. Circadian rhythms in bed rest: Monitoring core body temperature via heat-flux approach is superior to skin surface temperature. Chronobiol Int. 2017;34:666–676. doi: 10.1080/07420528.2016.1224241. PubMed DOI

Opatz O, et al. Temporal and spatial dispersion of human body temperature during deep hypothermia. Br J Anaesth. 2013;111:768–775. doi: 10.1093/bja/aet217. PubMed DOI

Dahyot-Fizelier C, et al. Accuracy of Zero-Heat-Flux Cutaneous Temperature in Intensive Care Adults. Crit Care Med. 2017;45:e715–e717. doi: 10.1097/CCM.0000000000002317. PubMed DOI

Evron S, et al. Evaluation of the Temple Touch Pro, a Novel Noninvasive Core-Temperature Monitoring System. Anesth Analg. 2017;125:103–109. doi: 10.1213/ANE.0000000000001695. PubMed DOI

Kimberger O, et al. The accuracy of a disposable noninvasive core thermometer. Can J Anaesth. 2013;60:1190–1196. doi: 10.1007/s12630-013-0047-z. PubMed DOI

Mäkinen MT, et al. Novel Zero-Heat-Flux Deep Body Temperature Measurement in Lower Extremity Vascular and Cardiac Surgery. J Cardiothorac Vasc Anesth. 2016;30:973–978. doi: 10.1053/j.jvca.2016.03.141. PubMed DOI

Moore AD, et al. Peak exercise oxygen uptake during and following long-duration spaceflight. J Appl Physiol (1985) 2014;117:231–238. doi: 10.1152/japplphysiol.01251.2013. PubMed DOI

Norsk P, Asmar A, Damgaard M, Christensen NJ. Fluid shifts, vasodilatation and ambulatory blood pressure reduction during long duration spaceflight. J Physiol. 2015;593:573–584. doi: 10.1113/jphysiol.2014.284869. PubMed DOI PMC

Morrison, S. F. Central control of body temperature. F1000Res5 (2016). PubMed PMC

Kobayashi S, Okazawa M, Hori A, Matsumura K, Hosokawa H. Paradigm shift in sensory system—Animals do not have sensors. Journal of Thermal Biology. 2006;31:19–23. doi: 10.1016/j.jtherbio.2005.11.011. DOI

Romanovsky AA. Thermoregulation: some concepts have changed. Functional architecture of the thermoregulatory system. Am J Physiol Regul Integr Comp Physiol. 2007;292:R37–46. doi: 10.1152/ajpregu.00668.2006. PubMed DOI

Sessler DI. Temperature monitoring and perioperative thermoregulation. Anesthesiology. 2008;109:318–338. doi: 10.1097/ALN.0b013e31817f6d76. PubMed DOI PMC

Taylor NA, Tipton MJ, Kenny GP. Considerations for the measurement of core, skin and mean body temperatures. J Therm Biol. 2014;46:72–101. doi: 10.1016/j.jtherbio.2014.10.006. PubMed DOI

Nybo L, Secher NH. Cerebral perturbations provoked by prolonged exercise. Progress in neurobiology. 2004;72:223–261. doi: 10.1016/j.pneurobio.2004.03.005. PubMed DOI

Caputa M. Selective brain cooling: a multiple regulatory mechanism. Journal of Thermal Biology. 2004;29:691–702. doi: 10.1016/j.jtherbio.2004.08.079. DOI

Arend WP, Malyak M, Guthridge CJ, Gabay C. Interleukin-1 receptor antagonist: role in biology. Annual review of immunology. 1998;16:27–55. doi: 10.1146/annurev.immunol.16.1.27. PubMed DOI

Cartmell T, Luheshi GN, Hopkins SJ, Rothwell NJ, Poole S. Role of endogenous interleukin‐1 receptor antagonist in regulating fever induced by localised inflammation in the rat. The Journal of physiology. 2001;531:171–180. doi: 10.1111/j.1469-7793.2001.0171j.x. PubMed DOI PMC

Chouker, A. Stress challenges and immunity in space: from mechanisms to monitoring and preventive strategies (Springer Science & Business Media, 2011).

Cogoli A, Tschopp A, Fuchs-Bislin P. Cell sensitivity to gravity. Science. 1984;225:228–231. doi: 10.1126/science.6729481. PubMed DOI

Crucian B, et al. Immune system dysregulation occurs during short duration spaceflight on board the space shuttle. J Clin Immunol. 2013;33:456–465. doi: 10.1007/s10875-012-9824-7. PubMed DOI

Reitz G, et al. Astronaut’s organ doses inferred from measurements in a human phantom outside the International Space Station. Radiation research. 2009;171:225–235. doi: 10.1667/RR1559.1. PubMed DOI

Oka T. Psychogenic fever: how psychological stress affects body temperature in the clinical population. Temperature. 2015;2:368–378. doi: 10.1080/23328940.2015.1056907. PubMed DOI PMC

Ley, W., Wittmann, K. & Hallmann, W. Handbook of space technology (John Wiley & Sons, 2009).

Rosenthal, R. & Rosnow, R. L. Contrast analysis: Focused comparisons in the analysis of variance (CUP Archive, 1985).

Bland JM, Altman DG. Calculating correlation coefficients with repeated observations: Part 1–Correlation within subjects. BMJ. 1995;310:446. doi: 10.1136/bmj.310.6977.446. PubMed DOI PMC

R Core Team (2016). R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. https://www.R-project.org/.

Bates, D., Mächler, M., Bolker, B. & Walker, S. Fitting linear mixed-effects models usinglme4. arXiv preprint arXiv:1406.5823 (2014).

Kuznetsova, A., Brockhoff, P. B. & Christensen, R. H. B. Package ‘lmerTest’. R package version2 (2015).

Bakdash JZ, Marusich LR. Repeated Measures Correlation. Front Psychol. 2017;8:456. doi: 10.3389/fpsyg.2017.00456. PubMed DOI PMC