When the Heat Sends Mental Health Issues to the Emergency Department

When temperatures soar, the risks most commonly mentioned and considered most concerning tend to be dehydration, heat exhaustion and heatstroke. However, the consequences of a heatwave do not end with physical health. Research published in July 2026 in Nature Health points to another, less visible effect: during prolonged periods of extreme heat, the likelihood of hospitalization for mental and behavioral disorders also increases.

The study analyzed 2,618,307 hospital admissions recorded during the hottest seasons between 2000 and 2019 across 852 locations in Brazil, Canada, Chile and New Zealand. The researchers used as their main reference a heatwave defined by an average daily temperature above the 97.5th percentile for each location for at least four consecutive days. Under that definition, the risk of hospitalization for mental health problems increased by 3.3% on the day of exposure and by 5.6% when that day and the following eight days were considered cumulatively.

It Is Not Just About Feeling More Irritable in the Heat

Spending several days sleeping poorly, coping with high temperatures or changing daily routines can cause fatigue, irritability or difficulty concentrating even in people without a pre-existing disorder. But the study did not simply measure subjective discomfort. It analyzed hospital admissions, in other words, situations serious enough to require hospital care.

In addition, the increase was not limited to a single disorder. When the different types were analyzed separately, the researchers also found a relationship between extreme heat and hospital admissions for anxiety, dementia, bipolar disorder, schizophrenia and substance use-related problems. This does not mean that a heatwave can cause these conditions on its own. The study identifies a temporal association: when extreme heat persists for several days, certain episodes of deterioration or crisis may become more likely.

That distinction matters. Mental health depends on biological, psychological, social and environmental factors that interact with one another, and temperature can act as an additional source of strain for people who are already in a vulnerable situation, without necessarily being the underlying cause of the problem.

What Can Happen in the Body

Keeping body temperature stable requires a constant physiological response. When it is hot, the body increases blood flow to the skin and uses sweating to release heat. If exposure continues, especially when night-time temperatures also prevent adequate recovery, this strain can contribute to dehydration, electrolyte imbalances, fatigue and changes in cognitive functioning.

On top of this comes lack of sleep. A 2026 review of night-time heat in middle-aged and older adults found that higher temperatures at night are consistently associated with shorter total sleep duration, lower sleep efficiency and more awakenings. Sleeping badly for one night may simply be uncomfortable; repeated poor sleep can affect emotional regulation, attention and the ability to cope with stress.

For that reason, the factors that may help explain the rise in hospital admissions should not be reduced to a single mechanism. Heat can combine physiological stress, lack of rest, changes in routine, greater isolation or difficulty accessing cool spaces. In people with certain mental health problems, this combination can contribute to a previously stable condition becoming harder to manage.

Medication Also Matters

There is another particularly relevant issue for people receiving treatment with psychotropic medication. In 2025, the Spanish Agency for Medicines and Health Products and the Commissioner for Mental Health published specific recommendations on their use during periods of high temperatures. Some medications can affect the body's ability to regulate temperature, sweating or the perception of heat-related discomfort.

Antipsychotics, for example, can interfere with thermoregulation and sweating. Some antidepressants can also alter sweating, while dehydration can increase the risk of lithium toxicity, because the loss of water and sodium can cause the kidneys to excrete less of the medication, increasing its concentration in the blood. This does not mean that treatment should be stopped when a heatwave arrives. On the contrary, any change should only be made under medical guidance. What matters is understanding the risk, staying properly hydrated and seeking advice if unusual symptoms appear.

The relationship between treatment and temperature also helps explain why two people exposed to the same environment may not respond in the same way. Age, pre-existing illnesses, medication, personal independence and housing conditions can significantly affect the ability to adapt.

Who Appears to Be More Vulnerable

During the study, a stronger association was found among people aged 60 and over. More pronounced effects were also observed in areas with lower population density and in places where travel time to healthcare services was longer. These findings are a reminder that vulnerability to heat does not depend solely on the temperature itself.

Living alone, having limited mobility, lacking adequate cooling or being far from healthcare services can make it harder to respond when physical or mental health begins to deteriorate. The World Health Organization specifically identifies older adults, people with chronic illnesses and those taking certain medications among the groups that require particular attention during periods of high temperatures.

Preparing for the Mental Health Effects of Heat Is Also Part of Heatwave Planning

General recommendations during a heatwave usually focus on drinking water regularly, avoiding the hottest hours of the day, keeping the home as cool as possible and reducing intense physical activity. For people with mental health problems, these measures may require additional planning: making sure medication is taken correctly, ensuring access to fluids and cool spaces, watching for noticeable changes in behavior and keeping in contact with people who spend many hours alone.

It is also important to distinguish ordinary heat-related discomfort from a warning sign. Severe confusion, seizures, loss of consciousness or a very high body temperature may be signs of heatstroke and require urgent medical attention. Likewise, a severe or sudden deterioration in a person's mental state should be medically assessed.

The research published in Nature Health does not show that heat is the direct cause of every hospitalization; however, its scale and its analysis of hundreds of locations reinforce an idea that is becoming increasingly important in public health: heatwaves do not affect the body only in the ways we have traditionally imagined.

When several days of extreme temperatures translate into greater hospital demand for mental health problems, preparing for the heat also means looking beyond heatstroke. Protecting sleep, taking vulnerability factors into account and anticipating possible deterioration can help reduce risks and identify sooner when someone needs medical care.