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In a controlled trial of 18 adults ages 67 to 84, researchers found that some measures of physiological strain persisted into the morning after daytime indoor heat exposure, despite cooling the room overnight to 26°C. The study supports guidance to keep indoor temperatures at or below 26°C during extreme heat, but that limit does not guarantee protection for every older adult.
A controlled trial of 18 adults ages 67 to 84 found that some signs of heat-related physiological strain remained the morning after daytime indoor temperatures reached 34°C, even when the room cooled to 26°C overnight. The University of Ottawa researchers say the findings suggest overnight cooling alone may not fully offset repeated daytime heat exposure during a multiday heat wave.
Participants from the Ottawa–Gatineau region spent three days and two nights in a climate-controlled apartment-style chamber. During the simulated heat-wave condition, the chamber reached 34°C (93°F) during the day and cooled to 26°C (79°F) overnight. In a comparison session, it stayed at 26°C throughout the day and night. The study was published in The Lancet Planetary Health.
On hot days, participants’ peak core body temperature averaged 37.8°C (100.0°F), compared with 37.4°C (99.3°F) under the cooler condition. Their heart rates averaged five beats per minute higher. Researchers also reported higher blood markers associated with kidney and gastrointestinal stress, lower blood pressure and cardiac autonomic function, and small declines in balance and cognitive test performance.
Most measured responses moved toward baseline overnight, but recovery was incomplete: lower morning systolic blood pressure and elevated kidney-stress markers remained evident the following mornings. The researchers reported no adverse events during the study. They cautioned that a stable core temperature from one day to the next did not mean all physiological effects had resolved.
Why Overnight Cooling May Fall Short
The findings matter because older adults may spend a heat wave indoors without access to air conditioning, yet indoor spaces can remain hot enough to expose people to sustained heat. The trial suggests that simply cooling a home at night may not erase all effects of repeated daytime overheating. It adds evidence that heat risk cannot be judged by core temperature alone: blood pressure and kidney-stress measures remained affected even when body temperature did not keep rising day by day.
The study supports maintaining indoor temperatures at or below 26°C (79°F) throughout the day and night during extreme heat, rather than relying on nighttime cooling alone. That figure is an upper limit in guidance, not a guarantee that every older adult is protected at that temperature. The researchers’ results describe measured responses in a small, controlled trial; they do not establish the health outcome for every person or home.
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How the Trial Extends Earlier Evidence
The University of Ottawa team designed the experiment to track older adults through both daytime exposure and overnight recovery during a simulated multiday heat wave. The report describes it as the first controlled trial to examine those periods together in older adults. The chamber temperatures were selected to reflect indoor conditions recorded during real heat events in Canada and the United States, according to the study report.
The findings build on the team’s earlier work on 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, informed in part by that earlier research. The new trial offers evidence from repeated exposure, while also emphasizing that 26°C should not be understood as a temperature that guarantees safety for all older adults.
“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.”
— Glen Kenny, University of Ottawa professor and study author, as quoted in the Medical Xpress report
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Limits of the Small Simulation
The trial involved 18 people at one site, ages 67 to 84, and simulated three hot days under controlled conditions. The findings do not show how often these effects lead to illness in everyday settings, how long the morning changes persist after the heat exposure ends, or whether the same patterns apply to older adults with different health conditions. The report does not provide enough information to establish which cooling approaches outside the chamber would be most effective or accessible.
Researchers observed small declines in balance and cognitive test performance, but the supplied report does not specify their clinical significance or whether they affected participants’ daily functioning. No adverse events were reported. The study’s results should not be read as a diagnosis or as proof that any particular person will experience the same response.
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Applying Findings During Heat Events
The study adds to evidence supporting Health Canada’s advice to keep indoor temperatures at or below 26°C for older adults during extreme heat. The report does not announce a change to that guidance or set out a new public-health policy. It leaves open how households without effective cooling can meet the recommended limit during prolonged heat.
Further research would be needed to test the findings across larger and more varied groups of older adults, and to assess practical cooling strategies over longer heat events. For now, the researchers’ reported conclusion is limited but relevant: overnight cooling may not fully reverse some effects of repeated daytime indoor overheating.
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Key Questions
What did the study find about overnight cooling?
Cooling the chamber to 26°C overnight allowed most measured responses to move toward baseline, but lower morning systolic blood pressure and elevated kidney-stress markers remained.
How many people took part?
The trial included 18 adults ages 67 to 84 from the Ottawa–Gatineau region, with equal numbers of women and men.
What temperatures were tested?
In the heat-wave condition, the chamber was kept at 34°C during the day and 26°C overnight. In a comparison session, it stayed at 26°C day and night.
Does 26°C guarantee that an older adult is safe from heat effects?
No. The researchers described 26°C as an upper indoor temperature limit intended to reduce risk, not a guarantee of protection for every older adult.
Did the study report any adverse events?
No adverse events were reported during the trial. The small, controlled study does not establish how often similar physiological changes lead to illness in everyday conditions.
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