Splitting daily heat exposure into three sessions slows acclimation compared with one continuous session

A seven-day controlled experiment with 60 healthy adults tested whether breaking a fixed total heat-exposure time into three shorter daily sessions produced the same physiological adaptations as a single uninterrupted session of equal length. Participants who completed one continuous session each day showed progressive reductions in core and skin temperature over the week, while those whose exposure was divided into three shorter bouts showed little or no such adaptation.

What the study found

The divided-session group did not achieve the same degree of heat acclimation as the continuous-session group — this is the central finding. Participants in the continuous group showed measurable, statistically significant drops in rectal temperature at both the midpoint and the end of the seven-day protocol, with improvement continuing between those two checkpoints. The divided group's core temperature remained essentially flat across all three measurement days.

Skin temperature followed a broadly similar pattern. The continuous group showed surface temperature declining from the first to the middle and final assessment days, while the divided group showed no meaningful change until the very end of the week, and even then the shift was modest.

Women consistently registered higher core and skin temperatures than men throughout the protocol, a difference the researchers controlled for by standardizing ovarian-hormone phase among female participants. Importantly, this sex difference in baseline temperature did not appear to alter the rate or magnitude of adaptation within each scheduling condition — both men and women responded similarly to whichever schedule they were assigned.

The authors interpret these results as evidence that continuous heat stress is a more potent driver of physiological heat adaptation than the same total duration fragmented across a day, at least over a one-week window. They express this conclusion with moderate confidence, noting the controlled nature of the design while acknowledging that longer observation periods were not examined.

Limits worth knowing

The study ran for only seven days, leaving open whether the divided-session group might eventually close the adaptation gap over a longer protocol. All sessions took place in a controlled humid-heat chamber during walking exercise, which differs meaningfully from the dry, sedentary environment of a traditional sauna, limiting direct transferability of findings to that modality. The sample, while reasonably sized at 60 participants, was drawn from a single site and presumably a healthy, physically active population, so generalizability to other groups is uncertain. Funding sources are not noted in the available abstract. The study did not assess longer-term retention of adaptations after the protocol ended, nor did it examine performance, cardiovascular, or subjective outcomes beyond thermoregulatory markers.

About this entry. The Heartwood Review summarizes published, peer-reviewed research in its own words and links to the original source. It is not medical advice; consult a physician before beginning or changing any thermal or recovery practice. Findings summarized here describe the study cited, not any product sold by Teak & Marble Atelier. Corrections.