Many of us think of the house as the safe place during a heatwave.
You pull the curtains, run the air conditioning at night and wait for a cooler morning.
For older bodies, that routine may do less than it seems.
A new three-day heatwave trial in The Lancet Planetary Health found that cooling a home to 26°C overnight did not fully reverse the physiological strain of a hot day in adults aged 67 to 84.
Also read: New sleep apnea pill cuts breathing interruptions nearly in half
Markers of kidney stress stayed elevated and morning blood pressure stayed lower the next day, even after the night of cooling.
The 26°C ceiling is not an arbitrary number.
Health Canada sets it as the upper indoor temperature limit to reduce heat-related illness and death in adults aged 60 and over, in line with World Health Organization guidance.
The new finding challenges the idea that reaching that number only at night is enough.
Also read: How cottage cheese can affect your blood sugar
What the Ottawa team measured
Researchers at the University of Ottawa recruited 18 older adults for a simulated three-day heatwave in a climate chamber.
Volunteers were aged 67 to 84, with equal numbers of women and men.
During the day the apartment was held at 34°C. At night it cooled to 26°C. A comparison condition kept the room at 26°C around the clock.
Peak core body temperature averaged 37.8°C during the hot condition. In the cooler setup it averaged 37.4°C.
Also read: Giving your child a smartphone may raise their eating disorder risk
Heart rates ran about five beats per minute higher. Blood tests showed elevated markers of kidney and gastrointestinal stress.
Lower morning systolic blood pressure and elevated kidney stress markers remained the following mornings, even after the apartment had cooled overnight.
Why overnight cooling misses the point
Overnight recovery was incomplete. A body that looks fine on a thermometer may still be carrying the load of the day before.
The study points to a different standard.
Indoor temperatures that stay at or below 26°C across the day and the night substantially reduce physiological strain in older adults during extreme heat.
What it means for older adults at home
Air conditioning overnight is a start, but the trial suggests the cooler setting needs to carry through the day too.
Fans, shade, cool showers and a cooler room during peak heat all help the body stay below the ceiling.
Older adults living alone, or with heart, kidney or neurological conditions, should be checked on during heatwaves.
Also read: Eating less ultra-processed food could help with depression
Doctors routinely advise drinking regularly, limiting midday exertion and moving to a cooler space when indoor temperatures climb.
Research has shown that extreme heat may be harming the body in more ways than doctors previously realized, from the heart and kidneys to the brain.
Health Canada's guidance, released in June 2026, keeps the same 26°C ceiling for both day and night during extreme heat and warns against prolonged indoor temperatures above that threshold, especially overnight and across consecutive days.
This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.