Heat Really Does Feel Hotter After a Cold Season

Heat acclimation study shows how we lose heat tolerance throughout the winter months

Baltimore (April 25, 2025)—Ever notice how going for a run on an 80-degree day feels scorching in the spring but comfortable at the end of August? New research shows that the reason for this is that we gradually lose our tolerance for heat during cold seasons, but it’s possible to regain it with a brief regimen of hot and humid workouts. Researchers will present their work this week at the 2025 American Physiology Summit in Baltimore. The Summit is the flagship annual meeting of the American Physiological Society (APS).

The study, funded by the U.S. Department of Defense, is part of a project to identify ways to enhance tolerance to exercise and heat stress. The outcomes of this area of research can be used to better prepare soldiers to fight in hot conditions or reduce heat-related risks for workers in fields like construction and farming.

“Heat acclimation studies are important because they lay the foundation to help guide strategies for ensuring safety and improving performance of individuals who encounter environmental heat stress during physical activity or labor,” said the study’s first author Michael R. Szymanski, PhD, a postdoctoral fellow working in the laboratory of Elaine C. Lee, PhD, in the Department of Kinesiology at the University of Connecticut. “With climate change, our work has become relevant to diverse populations, including those who are more susceptible to heat stress.”

During heat acclimation studies, volunteers come to a lab and exercise in hot and humid conditions repeatedly over the course of up to two weeks. By the end of this regimen, participants show performance benefits and enhanced heat tolerance thanks to adaptations such as reduced body temperature, reduced heart rate and increased sweat. They also report feeling more comfortable and less exhausted when exercising in the heat.

Researchers typically conduct these studies in the winter to avoid conflating the effects of laboratory heat acclimation from the natural acclimation our bodies experience in the summer, but it is unclear just how long our bodies retain heat tolerance after the summer. For the new study, researchers compared laboratory results from 42 volunteers who completed heat acclimation studies in the early, mid and late winter in New England. They found that participants in all three groupings had a similar level of heat tolerance before the acclimation regimen, but those who underwent the regimen in the later winter—February and March—saw significantly greater improvements in adaptations associated with body temperature, heart rate and sweat responses.

“Our study highlights the importance of how the time of year can influence adaptations associated with heat acclimation and thermoregulatory research,” said Szymanski. “As it gets warmer throughout the summer months, our bodies gradually adapt to the heat. The adaptations are lost as it becomes cooler out and you are no longer being exposed to the heat.”

The findings can help to guide future research protocols and hone approaches for enhancing performance in the face of heat stress. The researchers plan to further study how long heat tolerance adaptations last after building up naturally or being induced through a heat acclimation regimen.

NOTE TO JOURNALISTS: The American Physiology Summit will be held April 24–27, 2025, in Baltimore. To schedule an interview with the researchers, conference organizers or presenters, or to request abstract A-1365, “Despite Living in a Cold Region, the Months of Testing Matter for Thermoregulatory Research,” contact APS Media Relations or call 301.634.7314. Find more highlights from the meeting in our Newsroom.

Physiology is a broad area of scientific inquiry that focuses on how molecules, cells, tissues and organs function in health and disease. The American Physiological Society connects a global, multidisciplinary community of more than 10,000 biomedical scientists and educators as part of its mission to advance scientific discovery, understand life and improve health. The Society drives collaboration and spotlights scientific discoveries through its 16 scholarly journals and programming that support researchers and educators in their work.