Hot Summer Nights Pose Growing Health Risk in the U.S., Say Climate Experts

Climate scientists have raised the alarm about increasingly hot summer nights, describing them as an alarming consequence of climate change and a serious public health concern.

When nighttime temperatures do not drop significantly compared to the daytime highs, the human body struggles to cool down. According to Climate Central, a nonprofit organization that analyzes and reports on climate science, this poses a growing danger.

Extreme heat is now the leading cause of weather-related deaths in the United States, surpassing other extreme events like hurricanes, floods, and tornadoes, according to the National Weather Service.

This week alone, over 200 million people across the United States — from South Dakota to Florida and along the East Coast to Boston — are under extreme heat alerts. Major cities, including New Orleans, Memphis, St. Louis, Omaha, Des Moines, Savannah, Raleigh, Charleston, and Sioux Falls, have issued heat warnings. In many of these areas, afternoon temperatures are expected to soar by more than 10 degrees Fahrenheit — and the danger continues well after sunset.

 A Dangerous Trend in Nighttime Temperatures

Between 1970 and 2024, Climate Central analyzed average summer nighttime temperatures in 241 locations across the United States. Nearly all locations saw an increase, with an average rise of 3.1°F.

Looking ahead, the Environmental Protection Agency (EPA) projects that warm summer nights will become increasingly common in the coming decades. In many parts of the country, nighttime temperatures are expected to stay above 70°F more frequently.

Health Risks from Night Heat

The EPA warns that the most serious health effects of heatwaves are often linked to elevated nighttime temperatures.

The Centers for Disease Control and Prevention (CDC) echoes this concern, noting that warm night air places additional stress on the body, which needs cooler temperatures to regulate its internal heat.

Without this crucial overnight cooling, high-risk groups — including the elderly, children, and people with preexisting medical conditions — face increased health threats.

Prolonged exposure to hot evening temperatures is also associated with poorer sleep quality, which can impair immune function, worsen mental health, and increase the risk of chronic diseases like heart conditions.

While daytime heat often draws public attention, insufficient nighttime cooling worsens the effects of extreme heat. Night cooling is critical not just for the human body, but also for infrastructure and ecosystems to recover after a hot day.

 Urban Areas and the Heat Island Effect

Urbanization worsens the problem. Cities — filled with heat-retaining concrete and asphalt — are more vulnerable to rising nighttime temperatures, especially as green spaces are replaced with buildings and roads, according to the EPA.

Compared to surrounding rural areas, urban regions are hotter, especially at night, a phenomenon known as the “urban heat island effect.”

The ongoing rise in greenhouse gas concentrations and increased humidity are major contributors to rising nighttime heat. As the atmosphere warms, it holds more water vapor, which acts like an insulating layer, trapping heat and significantly limiting nighttime cooling, as explained by the National Oceanic and Atmospheric Administration (NOAA).