Sleeping in too-warm of a room can lead to potentially dangerous complications in heart function, particularly in older adults.
A new study of older Australians throughout a summer season found that once nighttime bedroom temperatures exceeded 75 degrees Fahrenheit (24 degrees C), there was a measurable shift in their autonomic nervous system - a network of nerves that handles unconscious tasks like heartbeat and breathing.
Heat places extra demands on the cardiovascular system by forcing it to work harder to cool the body. Heart rate increases by about 10 beats per minute with every degree of increased body temperature.
When someone is in a warm environment, blood vessels near the skin begin to dilate, a process called vasodilation, to release heat. The heart has to pump faster and harder to do this while maintaining normal blood pressure and circulation.
At night, this creates a perfect storm of physiological stress. Normally, sleep is when the body recovers; heart rate slows, blood pressure drops and the parasympathetic nervous system - the 'rest and digest' mode - takes over. But in a warm room, the body does not have that opportunity.
The new study found that when nighttime bedroom temperatures exceeded 75 degrees, older adults' nervous systems remained stuck in a higher-alert state. Heart rate increased and heart rate variability, a key marker of recovery, dropped.
Over time, this nightly strain can accumulate, potentially raising the risk of cardiovascular events like heart attack and stroke in vulnerable individuals, such as older people.
However, the latest research found that turning the thermostat below 75 degrees could mitigate this risk.
75 degrees Fahrenheit was the ideal sleep temperature threshold for heart health, researchers found. Once nighttime bedroom temperatures exceeded this, there was a measurable shift in their autonomic nervous system, which controls unconscious tasks like heartbeat and breathing.
When the heart must work harder by pumping faster and maintaining blood pressure while blood vessels dilate to shed heat, the system is under significant stress.
For older adults or those with underlying conditions, this can trigger a cascade of events.
The combination of increased heart rate, sympathetic nervous system takeover and reduced heart rate variability—a marker of poor recovery—deprives the cardiovascular system of its overnight rest period.
Over time this sustained strain can elevate blood pressure, promote arterial stiffness and increase the likelihood of clot formation or plaque rupture—events that directly precede heart attacks and strokes.
The study's authors point out that a warming planet, with its increase in uncomfortably hot nights, could make this kind of cardiovascular strain a growing concern.
Dr Fergus O'Connor from Griffith's School of Allied Health, Sport and Social Work at Griffith University, said: 'When the human body is exposed to heat, its normal physiological response is to increase the heart rate. The heart is working harder to try and circulate blood to the skin surface for cooling.
'However, when the heart works harder and for longer, it creates stress and limits our capacity to recover from the previous day's heat exposure.'
In the study, published in BMC Medicine, 47 people age 65 and up were monitored while they slept from November 2024 to March 2025 using high-tech wearables, though the researchers did not name which one. Each person was monitored while they slept from 9pm to 7am.
The risk of heart strain climbed steadily with each degree of warmth: temperatures between 75 to 79 degrees F carried 1.4 times higher odds of stress-related heart changes, rising to two times at 79 to 82 degrees, and 2.9 times at 82 to 90 degrees.
While 75 degrees was the key threshold for protecting heart health during sleep in older adults, when nighttime bedroom temperatures exceeded that, researchers observed clinically meaningful changes in heart rate and heart rate variability, a key marker of autonomic nervous system recovery.
The risks of potential heart damage escalated progressively with each degree of warmth: temperatures between 75 to 79 degrees carried 1.4 times higher odds of stress-related heart changes, rising two times at 79 to 82 degrees and 2.9 times at 82 to 90 degrees.
The researchers said: 'In many countries, heatwaves are typically characterized solely by elevated daytime temperatures, with little consideration given to elevated nighttime temperatures.'
Their findings 'reinforce the importance of quantifying not just extreme daytime highs but also elevated nighttime temperatures.'
It is likely that the cumulative effects of living through many hot days are compounded by sleeping on hot nights.
Dr O'Connor added: 'Climate change is increasing the frequency of hot nights, which may independently contribute to cardiovascular morbidity and mortality by impairing sleep and autonomic recovery.'
The researchers noted a critical gap in public health guidance. While the World Health Organization (WHO) recommends a maximum daytime indoor temperature of 79 degrees, there are currently no equivalent guidelines for nighttime conditions.
This matters because climate change is increasing the frequency of hot nights, which may independently contribute to cardiovascular strain by impairing sleep and limiting the body's overnight recovery.
They said: 'Importantly, climate projections forecast a shift wherein, by the year 2100, a larger proportion of heat-related deaths will result from hot nights rather than hot days.'