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In a controlled University of Ottawa study of 18 adults ages 67 to 84, repeated daytime indoor heat was followed by incomplete overnight recovery, even when the room cooled to 26°C. The findings indicate that a stable core temperature does not necessarily mean the body has recovered; the small laboratory trial does not establish risks for every older adult or household.
A University of Ottawa study found that some signs of heat-related strain persisted into the morning in older adults after repeated daytime exposure to a simulated heat wave, despite overnight cooling to 26°C (79°F). The small controlled trial suggests that nighttime cooling alone may not fully reverse the effects of a hot home during the day.
The trial included 18 adults ages 67 to 84 from the Ottawa–Gatineau region, with equal numbers of women and men. Participants stayed in a studio apartment-style climate chamber for three consecutive days and two nights. During the heat-wave condition, the chamber reached 34°C (93°F) by day and cooled to 26°C (79°F) overnight. In a comparison session, it remained at 26°C throughout.
On hot days, participants’ peak core temperature averaged 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. Researchers also reported higher blood markers of 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 were still evident the following mornings. The researchers reported no adverse events during the trial. The findings were published in The Lancet Planetary Health in a paper by Robert D. Meade and colleagues.
Why Daytime Cooling Still Matters
The findings matter for older adults who spend heat waves in homes that stay warm through the day. Being indoors does not necessarily prevent heat strain if indoor temperatures remain high, and a cooler night may not erase all effects of repeated daytime exposure. That has practical relevance in places where homes lack air conditioning or cannot be kept cool during a prolonged hot spell.
The study also cautions against relying on a single measure. Participants’ core temperatures did not keep rising day after day, yet some other signs of strain remained the next morning. As senior researcher Glen Kenny explained, a stable body temperature did not mean full recovery. The results are physiological measurements from a small controlled trial, not evidence that every older person will experience the same response or a specific health outcome.
The report says the findings support attention to Health Canada guidance released in June 2026 that recommends an upper indoor temperature limit of 26°C (79°F) for older adults during extreme heat. The research team stresses that this is a risk-reduction limit, not a guarantee of protection for every person.
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From One Hot Day to Three
Earlier work by the University of Ottawa team examined responses to a single day of heat exposure and helped inform the Health Canada recommendation, according to the report. The new trial extended that research by tracking participants across repeated hot days and overnight recovery in a simulated overheated home.
The chamber settings were selected to reflect indoor temperatures recorded during heat events in Canada and the United States, the report says. Researchers used a crossover design, comparing each participant’s responses under the heat-wave condition with a separate session held at 26°C day and night. This controlled setup allows direct physiological monitoring, but it does not reproduce every feature of living through a heat wave at home.
“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 Faculty of Health Sciences professor and research-unit director
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Limits of the Trial Findings
The study involved 18 participants at one site, in a controlled chamber over three days. The report does not establish whether the same patterns occur across larger and more varied groups of older adults, during longer heat waves, or in homes with different cooling, humidity, ventilation, and daily routines. It also does not show whether the measured changes led to illness or other clinical outcomes; no adverse events were reported in the trial.
Researchers observed several physiological and functional changes, but the report does not identify a temperature or cooling schedule that guarantees safety for every individual. 26°C is described as an upper limit to reduce risk, not a threshold that eliminates it. How particular health conditions, medications, or housing circumstances affect recovery was not answered by the reported findings.
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Applying the Findings to Homes
The study adds evidence for keeping indoor temperatures at or below the Health Canada upper limit during sustained extreme heat, rather than assuming overnight cooling will fully compensate for daytime overheating. For readers, the immediate relevance is to watch indoor conditions as well as outdoor forecasts, especially when an older adult’s home lacks adequate cooling.
The report does not announce a follow-up trial or a new change to guidance. Further research would be needed to test the findings in larger groups and in real homes over longer heat events, and to clarify how individual health and housing factors shape recovery. Until then, the study supports caution: one stable temperature reading or a cooler night cannot by itself show that heat-related strain has resolved.
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Key Questions
What did the study find about overnight cooling?
Most measured physiological responses moved toward baseline overnight, but lower morning systolic blood pressure and elevated kidney-stress markers remained after repeated daytime heat, even when the chamber cooled to 26°C (79°F).
Who took part in the trial?
The trial included 18 adults ages 67 to 84 from the Ottawa–Gatineau region, with equal numbers of women and men.
Did participants’ body temperatures rise each day?
No. Researchers said core temperature did not worsen from day to day, even though other measures of strain persisted. The authors cautioned that stable core temperature does not necessarily mean full recovery.
Does 26°C guarantee protection during a heat wave?
No. The report describes 26°C (79°F) as an upper indoor temperature limit intended to reduce risk, not a temperature that guarantees protection for every older adult.
Can the results be applied to every older adult?
The trial was small and conducted in a controlled chamber, so it does not establish that every older adult will respond in the same way. The study also did not determine how longer heat waves, individual health conditions, or different home environments affect recovery.
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