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In a controlled trial of 18 adults ages 67 to 84, some measures of physiological strain remained the morning after repeated daytime exposure to indoor heat, despite overnight cooling to 26°C. The researchers say the findings support keeping indoor temperatures at or below that level during heat events, but it is an upper limit intended to reduce risk, not a guarantee of protection.
A controlled University of Ottawa study found that some signs of physiological strain remained the morning after older adults experienced repeated daytime heat indoors, even when the room cooled overnight to 26°C (79°F). The trial, published in The Lancet Planetary Health, followed 18 adults ages 67 to 84 through a three-day simulated heat wave and suggests that nighttime cooling alone may not fully reverse the effects of daytime overheating.
In the heat-wave condition, researchers kept a studio-apartment-style climate chamber at 34°C (93°F) during the day and cooled it to 26°C overnight for two nights. In a separate comparison session, the apartment stayed at 26°C throughout the day and night. The study involved 18 participants from the Ottawa–Gatineau region, with equal numbers of women and men.
On hot days, participants’ average peak core temperature was 37.8°C (100.0°F), compared with 37.4°C (99.3°F) in the cooler condition. Their heart rates averaged five beats per minute higher. Blood tests showed higher circulating markers of kidney and gastrointestinal stress; researchers also observed lower blood pressure and cardiac autonomic function, as well as small declines in balance and cognitive test performance.
Most measured responses moved back toward baseline overnight, but recovery was incomplete. Lower morning systolic blood pressure and elevated kidney-stress markers were still evident the following mornings. The researchers reported no adverse events during the trial.
Why Nighttime Cooling May Fall Short
The findings matter because a person can remain indoors during a heat wave and still face high temperatures if a home lacks effective cooling. The study suggests that cooling only overnight may not erase all effects of repeated daytime heat exposure for older adults, even when the chamber reaches 26°C at night.
The results also caution against relying on core body temperature alone to judge heat strain. Participants’ temperatures did not keep rising from one hot day to the next, but some other measures had not returned to baseline by morning. The researchers say this means a stable temperature should not be taken as proof of full recovery.
The trial supports attention to indoor temperatures across the full day during sustained heat. It does not establish that 26°C is safe for every older adult or show that every person will respond in the same way. The researchers describe that level as an upper limit intended to reduce risk, not a guarantee of protection.
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How the Three-Day Trial Worked
The team used a controlled climate chamber designed to resemble a small apartment, allowing it to monitor participants during both daytime exposure and overnight recovery. Researchers describe the trial as the first controlled study to track older adults across both parts of a simulated multiday heat wave. The hot and overnight temperatures were selected to reflect indoor conditions recorded during real heat events in Canada and the United States, according to the study report.
The work builds on earlier research by the University of Ottawa team examining a single day of heat exposure. Health Canada issued guidance in June 2026 recommending an upper indoor temperature limit of 26°C for older adults during extreme heat, with the earlier research helping inform that advice. The new trial adds evidence about repeated daytime heat and overnight recovery, rather than evaluating an entire heat-wave response in people’s homes.
“Physiological strain persisted across each day of the heat wave, and some effects did not fully resolve overnight, even when the apartment cooled to 26°C (79°F).”
— Glen Kenny, University of Ottawa professor and director of the Human and Environmental Physiology Research Unit
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Limits of the Evidence
The trial included 18 people at one site, and researchers studied them in a controlled chamber rather than observing a broad group of older adults in their own homes. The results therefore show how this small sample responded under the tested conditions; they do not establish the effects for all older adults, housing types or heat-wave patterns.
The report does not establish whether the measured changes lead to illness or other clinical outcomes during real-world heat events, nor how different cooling schedules might affect recovery. It also does not show that 26°C fully prevents heat strain. The researchers reported no adverse events, but the study was not a test of every possible exposure or individual health condition.
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Further Research on Home Cooling
The published study adds evidence to existing guidance, but the supplied report does not identify a specific next trial or a timetable for further research. The researchers’ results point to questions for future work: how physiological strain develops in larger and more varied groups of older adults, how it relates to health outcomes, and which home-cooling approaches best reduce exposure across multiple hot days.
For now, the study’s authors say maintaining indoor temperatures at or below 26°C throughout the day and night can substantially reduce physiological strain in older adults during extreme heat. They also stress that the level is not a guarantee of safety for every person.
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Key Questions
What did the study find about overnight recovery?
Most measured responses moved toward baseline after the chamber cooled overnight to 26°C (79°F), but recovery was incomplete. Lower morning systolic blood pressure and elevated kidney-stress markers remained evident the following mornings.
How hot was the simulated heat wave?
The apartment-style chamber was kept at 34°C (93°F) during the day and cooled to 26°C (79°F) overnight. A comparison session kept it at 26°C throughout day and night.
Does a stable core temperature mean an older adult has fully recovered?
Not according to the study’s findings. Core temperature did not rise progressively from day to day, but some other measures of strain persisted into the morning. The researchers caution that temperature alone may not show the full physiological response.
Does the study prove that 26°C protects every older adult?
No. The researchers describe 26°C as an upper indoor limit intended to reduce risk, not a temperature that guarantees protection for every person. The trial involved 18 participants in a controlled setting.
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