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In a controlled trial of 18 adults ages 67 to 84, some signs of physiological strain remained the morning after simulated hot days, even when the apartment cooled to 26°C overnight. Researchers say the results support limiting indoor heat throughout a heat wave, but the small, single-site study does not show how every older adult would respond.
Some signs of heat-related physiological strain remained the next morning in older adults after repeated hot days, even when a simulated apartment cooled overnight to 26°C (79°F), according to a University of Ottawa study published in The Lancet Planetary Health. The controlled trial involved 18 adults ages 67 to 84 and suggests that nighttime cooling alone may not fully offset repeated indoor overheating during a heat wave.
Researchers monitored participants in an apartment-style climate chamber over three days and two nights. During the simulated heat-wave session, the chamber reached 34°C (93°F) by day and cooled to 26°C overnight. In a separate comparison session, it stayed at 26°C throughout both day and night. The researchers said the simulated hot conditions reflect indoor temperatures recorded during real heat events in Canada and the United States.
On hot days, participants’ peak core body temperature averaged 37.8°C (100.0°F), compared with 37.4°C (99.3°F) in the cooler session. Their heart rates averaged five beats per minute higher. Blood tests showed higher circulating markers of kidney and gastrointestinal stress, while researchers also observed lower blood pressure and cardiac autonomic function, as well as small declines in balance and performance on cognitive tests.
Most measured responses recovered close to baseline overnight, the study reported, but recovery was not complete. Lower morning systolic blood pressure and elevated kidney-stress markers persisted into the following mornings. The researchers reported no adverse events during the trial. The results are from a small, single-site study and do not establish how common or severe these effects would be across older adults living in different conditions.
Why Nighttime Cooling May Fall Short
The findings matter because indoor heat can remain high during a multiday heat wave, particularly in homes without air conditioning. The study challenges the assumption that being indoors, or cooling a home overnight, necessarily means an older person has fully recovered from daytime heat exposure.
Lead researcher Glen Kenny said the results support keeping indoor temperatures at or below 26°C throughout the day and night during a sustained heat event. He also cautioned that this is an upper limit intended to reduce risk, not a temperature that guarantees protection for every older adult. The trial measured short-term physiological outcomes; it did not test whether the observed changes led to illness, hospitalization or other health outcomes.
One practical implication is that core body temperature alone may not capture all signs of heat strain. In the trial, it did not rise from one day to the next, while some other measures remained altered the next morning. The researchers said that a stable temperature should not be taken as proof that the body has fully recovered.
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How the Three-Day Heat Trial Worked
The study, by Robert D. Meade and colleagues, is described as a single-site randomized crossover trial. Participants from the Ottawa–Gatineau region completed both the heat-wave and cooler comparison conditions, allowing researchers to compare responses under different indoor temperatures. The group included equal numbers of women and men.
The work extends earlier research by the University of Ottawa team that examined a single day of heat exposure. The source report says that earlier work informed Health Canada guidance released in June 2026, which recommends an upper indoor temperature of 26°C for older adults during extreme heat. The new trial examined repeated daytime exposure and overnight recovery under controlled conditions, rather than tracking people in their own homes during an actual heat wave.
The article’s findings were published in The Lancet Planetary Health. While the chamber conditions were designed to reflect temperatures recorded indoors during real heat events, a controlled simulation cannot reproduce every feature of a household, including differences in ventilation, humidity, health status, medication use or access to cooling.
“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 study lead
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Limits of the Study Evidence
The trial included just 18 participants at one research site, so the findings may not represent all older adults. The supplied report does not provide enough information to determine whether responses differed by participants’ health conditions, medications, housing or other personal factors. It also does not establish whether the measured changes lead to clinical illness or how long any effects last beyond the study’s observation period.
The chamber simulated particular indoor temperatures and a short heat-wave sequence. How the results apply to longer heat events, different overnight temperatures, humidity levels or real-world cooling practices remains unclear. The study reported no adverse events, but that does not establish that the same exposure would be safe for other people or in other circumstances.
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Applying the Findings During Heat Events
The immediate public-health reference point cited by the researchers is Health Canada’s 26°C upper indoor temperature recommendation for older adults during extreme heat. The study authors say the results add evidence for keeping indoor temperatures at or below that level throughout a sustained event, rather than relying on nighttime cooling alone. The trial does not evaluate specific cooling programs or determine which interventions are most feasible for households.
Further research could examine larger and more varied groups of older adults, longer heat waves and outcomes in people’s homes. For now, the study’s next step for readers is not a new policy announcement but a clearer account of what nighttime recovery may leave unresolved. The researchers’ findings support continued attention to indoor temperatures across the full day, while the degree of risk for any individual remains dependent on circumstances the trial did not test.
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Key Questions
What did the study find about overnight cooling?
In the trial, most physiological measures recovered close to baseline overnight after the apartment cooled to 26°C (79°F), but recovery was incomplete. Lower morning systolic blood pressure and elevated kidney-stress markers remained evident the next mornings.
How hot was the simulated apartment during the day?
During the heat-wave condition, the apartment-style chamber was kept at 34°C (93°F) during the day and cooled to 26°C overnight. The comparison condition stayed at 26°C day and night.
Does a stable core temperature mean an older adult has recovered?
Not necessarily, according to the study’s researchers. Participants’ core temperatures did not keep increasing day by day, but some other measures of strain remained altered the following morning.
Does 26°C guarantee protection from heat effects?
No. The study lead described 26°C as an upper indoor temperature limit intended to reduce heat-related risk, not a guarantee of protection for every older adult.
How broadly do the findings apply?
The trial involved 18 adults ages 67 to 84 at one site in a controlled chamber. It provides evidence about responses under those conditions, but does not establish how every older adult would respond in a real home or during a longer heat wave.
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