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In a University of Ottawa laboratory study, 18 adults ages 67 to 84 experienced three simulated hot days, with the apartment cooled overnight to 26°C. Several measures of physiological strain did not fully return to baseline by morning, supporting guidance to limit indoor heat throughout a heat event, though the small study does not establish individual risk or a guaranteed safe temperature.
A controlled University of Ottawa study found that some signs of heat-related physiological strain persisted overnight in older adults after repeated hot days, even when the simulated apartment cooled to 26°C (79°F). The findings, published in The Lancet Planetary Health, suggest that cooling only at night may not fully offset daytime indoor overheating during a multiday heat wave.
The trial included 18 adults ages 67 to 84 from the Ottawa–Gatineau region, with equal numbers of women and men. Participants stayed in an apartment-style climate chamber for three consecutive days and two nights. In the heat-wave condition, daytime indoor temperature was held at 34°C (93°F) and the room cooled to 26°C overnight. In a separate comparison session, it remained at 26°C day and night. The researchers said these simulated hot indoor conditions reflect temperatures recorded during real heat events in Canada and the United States.
During the hot-day sessions, participants’ peak core temperature averaged 37.8°C (100.0°F), compared with 37.4°C (99.3°F) in the cooler condition. Their average heart rate was five beats per minute higher during the heat condition. Blood tests also showed higher circulating markers of kidney and gastrointestinal stress, while researchers recorded lower blood pressure and cardiac autonomic function, as well as small declines in balance and cognitive test performance.
Cooling overnight allowed most measured responses to move close to baseline, but recovery was incomplete. Lower morning systolic blood pressure and elevated kidney-stress markers were still evident on following mornings, according to the report. The study recorded no adverse events. Its results describe responses in a small, monitored laboratory trial and do not establish how every older adult would respond at home.
Why Daytime Cooling Matters
The findings add evidence that indoor heat can strain the body even without outdoor exposure, particularly when a home remains hot through the day. Older adults may face greater exposure where homes lack air conditioning or cannot be cooled adequately. A cooler night may provide some recovery, but this trial found it did not erase every measured effect of repeated daytime heat.
The results also caution against relying on core temperature alone to judge the effects of heat. Researchers observed that participants’ core temperatures did not rise progressively from one day to the next, while some other measures remained abnormal the following morning. That distinction matters because a stable temperature reading did not mean all measured physiological responses had returned to baseline.
The work supports attention to indoor temperatures across the full heat event, rather than only overnight. However, the researchers describe 26°C as an upper limit intended to reduce risk, not a temperature that guarantees protection for every older person.
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From Single-Day Heat to Recovery
The new trial extends earlier work by the University of Ottawa team, which examined the effects of 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; the supplied report says the guidance was informed in part by the team’s earlier research. The current study examined repeated daytime exposure and overnight recovery rather than a single hot day.
Researchers used a controlled chamber to reproduce an overheated home environment and monitor participants through both daytime exposure and recovery. The study was a single-site randomized crossover trial, meaning participants took part in heat and cooler comparison conditions. The authors characterize it as the first controlled trial to follow older adults across both daytime heat and overnight recovery during a simulated multiday heat wave.
Lead researcher Glen Kenny, a professor in the University of Ottawa’s Faculty of Health Sciences and director of its Human and Environmental Physiology Research Unit, said many homes without adequate cooling can remain hot for days. The study’s controlled setting helps compare conditions, but it cannot capture every difference among homes, heat waves or individual health circumstances.
“What we saw is that 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 researcher
Limits of the Trial Evidence
The study involved 18 participants at one site, so the reported averages should not be taken as predictions for every older adult. The supplied report does not provide enough detail to establish how responses vary by existing health conditions, medication use, housing characteristics or different patterns of heat exposure. It also does not show whether the measured changes led to illness or other clinical outcomes; no adverse events occurred during the trial.
The study compared a particular simulated schedule—34°C by day and 26°C overnight—with a constant 26°C condition. It does not establish a precise temperature at which harm begins, nor that 26°C removes all risk. The researchers say it is an upper limit intended to reduce risk, and the effects of other cooling approaches were not detailed in the source report.
Applying the Findings During Heat
The published study adds evidence for maintaining cooler indoor conditions throughout a sustained heat event, in line with Health Canada’s 26°C upper-limit guidance for older adults. The report does not announce a change to that guidance or describe a new follow-up trial. Further research with larger and more varied groups could clarify how health status and real-world housing conditions affect overnight recovery.
For readers, the immediate takeaway from the researchers is that an indoor setting is not automatically cool or protective, and a night of cooling may not fully reverse strain after repeated hot days. The study does not offer individualized medical advice; its findings describe group responses under laboratory conditions, with protection varying among people.
Key Questions
What did the study find about overnight recovery?
Most measured responses moved close to baseline after the apartment cooled to 26°C overnight, but morning systolic blood pressure remained lower and kidney-stress markers remained elevated on following mornings.
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 did researchers simulate?
The heat condition used 34°C (93°F) during the day and 26°C (79°F) overnight. The comparison condition stayed at 26°C day and night.
Does the study show that 26°C guarantees safety?
No. The researcher described 26°C as an upper indoor temperature limit intended to reduce risk, not a temperature that guarantees protection for every older adult.
Did participants’ core temperatures rise each day?
No progressive day-to-day rise was observed, but some other measures of physiological strain persisted into the morning. The researchers said core temperature alone does not show whether the body has fully recovered.
Source: rss
