Patients arriving at hospitals during periods of extreme heat have long been associated with dehydration, heatstroke and cardiovascular emergencies. Increasingly, clinicians are also seeing another consequence: more people seeking urgent care for mental and behavioral disorders as prolonged heat strains both body and mind.
New research published this month in Nature Health provides some of the strongest multinational evidence yet that sustained heatwaves are associated with a measurable increase in hospitalizations for mental health conditions, adding to a growing body of research that links rising temperatures with psychological distress and increased demand for psychiatric care.
The study arrives as many parts of Europe and North America confront another summer of record-breaking temperatures. In recent weeks, hospitals have faced mounting pressure from heat-related illnesses while public health agencies have warned residents to avoid outdoor activity during the hottest hours of the day.
Researchers led by Monash University analyzed more than 2.6 million warm-season hospitalization records collected between 2000 and 2019 from 852 locations across Brazil, Canada, Chile and New Zealand. They found that sustained periods of extreme heat—defined as temperatures exceeding the local 97.5th percentile for at least four consecutive days—were associated with an increased risk of hospitalization for mental and behavioral disorders.
The increase was modest but consistent. The researchers reported elevated risks both on the day of exposure and during the following week, suggesting that prolonged heat can have lingering effects on mental health services. Older adults and people living in less densely populated areas experienced stronger associations, highlighting differences in vulnerability across communities.
The findings do not suggest that heat alone causes mental illness. Instead, researchers say extreme temperatures may aggravate existing psychiatric conditions through several pathways, including disrupted sleep, physiological stress, dehydration and challenges regulating body temperature. Certain psychiatric medications can also reduce the body's ability to dissipate heat, increasing vulnerability during prolonged hot weather.
Those mechanisms have become increasingly relevant as heatwaves grow more frequent and intense.
Across Europe, a powerful late-June heatwave pushed temperatures to record levels in several countries. According to mortality monitoring data reported by Reuters, more than 10,000 excess deaths were recorded during the period, most among older adults. Hospitals simultaneously reported surging emergency demand as health systems struggled to manage the consequences of extreme temperatures.
Health specialists say mental health often receives less attention during heat emergencies because public messaging typically focuses on preventing heatstroke and dehydration. Yet psychological effects may emerge alongside physical illness, particularly for people already living with anxiety disorders, schizophrenia, depression, dementia or substance-use disorders.
Children and adolescents may also face heightened risks.
Separate research from the University of Sydney found that hospital admissions for mental health conditions among young people increased during periods of extreme heat. The researchers reported elevated risks across a range of conditions, including depression, substance misuse and behavioral disorders, reinforcing concerns that climate-related heat affects people across the lifespan rather than only older adults.
Experts caution that social factors also influence how people experience heat.
Individuals without reliable air conditioning, those living alone, outdoor workers and people experiencing homelessness often have fewer opportunities to escape sustained high temperatures. Rural communities may face additional challenges because specialized mental health services are often located farther away, making timely treatment more difficult during emergencies.
Sleep disruption appears to be one of the most important pathways linking heat and psychological distress. When nighttime temperatures remain unusually high, the body has less opportunity to recover from daytime heat exposure. Researchers have warned that increasingly common "tropical nights" can impair sleep quality, concentration and emotional regulation while adding physiological stress that may compound existing psychiatric conditions.
The implications extend beyond individual patients.
Hospitals routinely prepare for predictable surges in heat-related physical illness during the summer, but the new evidence suggests psychiatric services may also require additional staffing and planning when prolonged heatwaves are forecast. The multinational study's authors argue that incorporating mental health into heat preparedness plans could help health systems anticipate increased demand before temperatures peak.
Public health agencies have increasingly framed extreme heat as a whole-of-health issue rather than simply an environmental hazard. Alongside advice to stay hydrated, seek cool environments and check on vulnerable neighbors, experts say clinicians should remain alert for worsening psychiatric symptoms among patients exposed to sustained heat.
Climate scientists expect extreme heat events to become more frequent and intense as global temperatures continue to rise, increasing the importance of understanding health effects that extend beyond heat exhaustion and cardiovascular disease.
For researchers studying the intersection of climate and health, the latest findings add another piece to a rapidly expanding picture: heatwaves are affecting hospital systems in ways that reach far beyond emergency treatment for overheating. They are also placing growing pressure on mental health services, underscoring that the consequences of a warming climate can be measured not only in temperatures and mortality statistics, but also in the quieter rise of people arriving at hospitals seeking psychiatric care when the heat refuses to break.
