Cold, Heat, and the Science of Human Adaptation: A Deep Dive into a New Systematic Review

Is the ice bath always the ultimate recovery tool? A new systematic review published in Frontiers takes a closer look at what cold and hot water immersion can actually do, revealing distinct effects on physical adaptation, sleep quality, hormonal responses, and cellular resilience.

The systematic review “Effects of post-exercise hot and cold-water immersion on performance recovery and physiological markers in athletes”, published on 3 July 2026 in Frontiers in Sports and Active Living, in the Elite Sports and Performance Enhancement section, was authored by Juan Bustos Carvajal, Elkin Pérez Hurtado, Mónica Bustos Carvajal, Diego Mauricio Celis Villamizar, and Florencio Arias Coronel, researchers primarily affiliated with the University of Pamplona in Colombia, and examines the available evidence on the effects of post-exercise hot and cold-water immersion on recovery and key physiological markers in athletes.

For practitioners of the Wim Hof Method, the cold is far more than a simple recovery trick. We step into the cold to train our minds and master our nervous systems. This perspective is closely aligned with a broader idea at the heart of cold exposure practices: that a controlled environmental stressor can become a stimulus for adaptation. Yet, in sports medicine, a debate continues to swirl around the physiological benefits of water temperature, with athletes questioning whether cold water immersion (CWI) or hot water immersion (HWI) is the superior choice for post-exercise recovery.

To address this, a systematic review published in the journal Frontiers evaluated randomized clinical trials up to December 2024. Researchers analyzed how active individuals and athletes respond to temperatures ranging from chilly cold (8–15 °C) to soothing hot (38–42 °C).

The Cold as a Teacher of Cellular Resilience

For practitioners of the Wim Hof Method, this concept of using cold exposure as a stimulus for adaptation will be familiar. The cold is not simply something to endure, but a controlled stressor that challenges the body and mind. While this review does not examine the Wim Hof Method itself, its findings offer an interesting perspective on some of the physiological adaptations associated with repeated cold exposure.

The researchers found that repeated cold water immersion over a four-week period significantly increased baseline levels of Heat Shock Protein 72 (HSP-72). As a cellular marker associated with thermal stress and proteostatic adaptation, HSP-72 suggests that repeated cold exposure can act as a meaningful physiological stimulus, prompting cells to adapt to thermal stress and maintain protein stability.

Beyond this cellular response, cold water immersion induces pronounced peripheral vasoconstriction, reducing femoral blood flow and lowering heart rate below resting levels. This thermal challenge places the body in an energy-demanding state as it works to maintain its core temperature. Cold immersion also significantly elevates excess post-exercise oxygen consumption (EPOC), reflecting the additional metabolic energy required for thermogenesis and the restoration of core temperature.

The Surprising Benefits of Hot Water Immersion

While cold exposure appears to promote specific cellular adaptations, hot water immersion emerged from the review with its own distinct benefits.

For individuals focused on rest and recovery, the hot bath may be a particularly useful tool. HWI was the only strategy in the review to show significant improvements in sleep quality and reductions in fatigue. By influencing thermoregulation, hot water immersion may help facilitate the transition toward sleep.

In addition to these effects, HWI was associated with a more favorable hormonal profile in some studies. Notably, repeated HWI over four weeks was associated with increased baseline resting testosterone levels. While this finding is interesting in the context of exercise adaptation, an increase in testosterone alone does not establish that HWI directly increases muscle repair or hypertrophy.

HWI also maintains vascular circulation through vasodilation. Over time, repeated HWI appeared more effective than cold immersion at reducing baseline levels of creatine kinase, a commonly used biomarker associated with muscle damage.

Strategic Timing and Biological Individualization

To get the most out of these thermal tools, timing and individual biology must be considered.

The review found that using CWI immediately before explosive exercise was associated with a 2.4% reduction in anaerobic power, potentially because lower muscle temperature can affect nerve conduction and muscle function. Conversely, HWI before exercise was associated with a 2.2% improvement in explosive power.

Timing may also matter after strength training. Because cold water immersion can attenuate some inflammatory and cellular signaling responses, repeated CWI immediately after resistance exercise may interfere with some of the processes involved in muscle hypertrophy. This does not mean that every cold exposure after strength training will reduce muscle growth, but it suggests that the timing of cold exposure should be considered in relation to the specific adaptation an athlete is trying to achieve. Lastly, biological sex differences are important. While much of the existing research has focused on men, studies involving women have shown distinct responses, including differences in creatine kinase following HWI. Differences in subcutaneous fat and hormonal physiology may contribute to these variations, highlighting the need for more individualized approaches to thermal exposure.

Rather than viewing cold and heat as competing recovery tools, the evidence points toward a more strategic approach. Water temperature can be used as a physiological dial, with different temperatures creating different challenges and adaptations. The goal is not simply to recover faster, but to understand how the body responds to stress and use that response intentionally to support recovery, adaptation, and resilience.