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In a controlled study of 18 adults ages 67 to 84, repeated daytime indoor heat exposure was followed by incomplete overnight recovery, even when the simulated home cooled to 26°C. Researchers reported lingering signs including lower morning systolic blood pressure and elevated kidney-stress markers; the trial does not establish individual health outcomes or a universally protective indoor temperature.
A controlled University of Ottawa study found that some signs of physiological strain persisted into the morning in older adults exposed to repeated daytime indoor heat, after the simulated home cooled overnight to 26°C (79°F). The findings, published in The Lancet Planetary Health, document incomplete overnight recovery in this study; they do not establish whether overnight cooling is sufficient to protect older adults during a multiday heat wave.
The trial included 18 adults ages 67 to 84, with equal numbers of women and men, from the Ottawa–Gatineau region. Participants stayed in a purpose-built studio apartment-style climate chamber at the University of Ottawa for three days and two nights. In the heat-wave condition, the chamber reached 34°C (93°F) during the day and cooled to 26°C overnight. A separate comparison session kept it at 26°C throughout the day and night. Researchers said these temperatures reflect indoor conditions recorded during real heat events in Canada and the United States.
During the hot-day condition, participants’ peak core temperature averaged 37.8°C (100.0°F), compared with 37.4°C (99.3°F) during the cooler condition. Their average heart rate was five beats per minute higher. Blood tests showed higher circulating markers associated with 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 physiological responses moved close to baseline overnight, but recovery was incomplete. Lower morning systolic blood pressure and elevated kidney-stress markers remained evident the following mornings. The research team reported no adverse events during the trial. The study assessed short-term responses under controlled conditions; it did not test whether these changes lead to specific illnesses or establish a temperature that protects every person. The team described the work as the first controlled trial to follow older adults through a full three-day heat wave, including overnight recovery. The findings were published in The Lancet Planetary Health.
Why Overnight Cooling May Fall Short
The findings are relevant to concerns about high indoor temperatures during heat events, including in homes without air conditioning, where older adults may be exposed to heat indoors. In this study, some measured effects remained the morning after repeated hot days, while most responses moved close to baseline overnight. The results describe the trial’s observations and do not establish whether overnight cooling is sufficient to protect older adults in other settings.
Researchers emphasized that a stable core temperature is not a complete measure of recovery: some participants’ temperatures did not rise further from one day to the next, while other measures remained altered the following morning. The trial supports attention to indoor conditions across the full day, but its small sample and laboratory setting mean it cannot predict an individual’s risk or show how much any particular cooling measure changes health outcomes.
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How the Heat-Wave Trial Was Run
The team describes the work as the first controlled trial to follow older adults’ physiological responses during both daytime exposure and overnight recovery across a simulated multiday heat wave. The participants stayed in a purpose-built chamber at the University of Ottawa, allowing researchers to control temperatures and compare the heat-wave condition with a cooler session.
The research builds on earlier work by the team examining a single day of heat exposure. Health Canada issued guidance in June 2026 recommending an upper indoor temperature limit of 26°C (79°F) for older adults during extreme heat. The agency’s advice was informed in part by earlier research. The new trial adds observations across repeated hot days and overnight recovery. The researchers describe 26°C as an upper limit intended to reduce risk, not a guarantee of protection for every older adult.
“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 research-unit director
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Limits of the Small Laboratory Study
The trial involved 18 participants at one site and simulated a specific pattern of indoor heat over three days. The study’s controlled setting and small participant group limit what it can establish about responses in other settings or groups. It also does not show whether the observed changes caused clinical illness or how long any residual effects lasted beyond the monitored period.
The study reports group-level measurements and observed changes. It did not determine a single indoor temperature that guarantees safety; the researchers describe 26°C as a limit intended to reduce heat-related risk.
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Applying Findings to Heat Guidance
The study adds evidence for public-health agencies and households considering how to reduce indoor heat exposure during sustained heat events. The researchers’ stated recommendation is to maintain indoor temperatures at or below 26°C (79°F) throughout the day and night where possible, while recognizing that this limit does not guarantee protection for everyone. The report does not announce a new guideline or a change to Health Canada’s June 2026 advice.
Further research would be needed to test the findings in larger groups and real homes, and to assess how different cooling strategies affect recovery and health outcomes. Until then, the trial’s direct conclusion is limited but relevant: in this small controlled sample, overnight cooling brought most responses close to baseline, while some measures of strain remained abnormal the next morning.
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Key Questions
What did the study find?
Some signs of physiological strain remained the morning after older adults experienced repeated hot days indoors, even when the room cooled to 26°C (79°F) overnight.
How many people took part?
The controlled trial included 18 adults ages 67 to 84 from the Ottawa–Gatineau region, with equal numbers of women and men.
Which effects persisted overnight?
Researchers reported lower morning systolic blood pressure and elevated blood markers associated with kidney stress. Most other measured responses moved close to baseline overnight.
Does 26°C guarantee protection from heat?
No. The researchers described 26°C as an upper indoor temperature limit intended to reduce heat-related risk, not a guarantee of protection for every older adult.
Did the study show that participants became ill?
The report says no adverse events were reported. It measured short-term physiological and functional changes and did not establish that those changes led to specific illnesses.
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