--- title: "When the Nights Won't Cool: The Rising Danger of Warm Summer Evenings" description: "During a heat wave, the headlines fixate on triple-digit afternoons. But scientists and doctors increasingly warn about a quieter danger: nights that never cool down. Overnight temperatures are rising faster than daytime highs, and in Los Angeles, many residents have no way to escape the heat when their bodies most need a break." category: "U.S." category_url: https://herald.la/category/us author: "Gabriela Soto" published: 2026-07-02T19:38:26.000Z updated: 2026-07-02T19:38:26.000Z canonical: https://herald.la/article/when-the-nights-won-t-cool-the-rising-danger-of-warm-summer-evenings tags: ["heat", "climate", "public health", "Los Angeles", "urban heat"] --- # When the Nights Won't Cool: The Rising Danger of Warm Summer Evenings During a heat wave, the headlines fixate on triple-digit afternoons. But scientists and doctors increasingly warn about a quieter danger: nights that never cool down. Overnight temperatures are rising faster than daytime highs, and in Los Angeles, many residents have no way to escape the heat when their bodies most need a break. Everyone watches the afternoon high during a heat wave. The number that may matter more for your health is the overnight low — and across the country, it is climbing. ## The night that doesn't cool The human body relies on cooler nights to shed the day's heat: to lower its core temperature, rest the heart and sleep. When the outdoor temperature stays high after dark, that recovery never fully happens, and the strain accumulates day after day. That is why prolonged heat waves, not single hot afternoons, drive the worst health outcomes. The trend is unmistakable. Across the United States, summer nighttime temperatures have risen about 3.1 degrees Fahrenheit since 1970, according to the research group Climate Central. And overnight lows are warming faster than daytime highs — roughly 1.45 degrees per century versus about 1.13 degrees, [NOAA reports](https://www.climate.gov/news-features/blogs/beyond-data/climate-change-rule-thumb-cold-things-warming-faster-warm-things), or about 20 percent quicker. ## Why nights warm faster The physics is straightforward. Greenhouse gases trap heat that the ground radiates back toward space, an effect that matters most at night, when there is no incoming sunlight to swamp it. So the warming shows up disproportionately in the overnight numbers. Cities make it worse. Concrete, asphalt and buildings soak up heat all day and release it slowly after dark — the "urban heat island" effect, which can add several degrees to nighttime temperatures and lingers longest in the small hours. Humid air compounds the problem, acting like a blanket that keeps warmth from escaping. In a sprawling, paved metropolis like Los Angeles, those forces can keep whole neighborhoods warm long after sunset. ## The health toll Doctors worry most about the heart. As the CDC notes, heat forces the cardiovascular system to work harder to push blood toward the skin for cooling, and warm nights can worsen heart failure and raise the risk of heart attacks and strokes among vulnerable people. Sleep suffers too, with lasting effects on health. And heat illness can escalate to heat stroke — a medical emergency, when body temperature climbs above 103 degrees — which the National Weather Service warns can be fatal without fast treatment. The burden is not shared evenly. Older adults are especially at risk; federal data attribute a large share of U.S. heat deaths to people 65 and older. Low-income households, many without reliable air conditioning, fare worst, and [USC researchers have found](https://today.usc.edu/urban-heat-waves-los-angeles-vulnerable-communities-usc-research/) that heat-related emergency room visits in Los Angeles cluster in lower-income neighborhoods with little tree cover — areas that can run several degrees hotter than leafier ones. Outdoor workers and people experiencing homelessness have the least escape of all. ## How to stay safe The practical advice is simple but matters most at night. If you have air conditioning, use it during overnight heat rather than saving it for the day. If you don't, spend the hottest hours in cooled public spaces — libraries, malls, cooling centers — and take a cool shower before bed. Drink water steadily instead of waiting to feel thirsty, skip alcohol, and check on elderly neighbors and relatives daily. Call 911 for signs of heat stroke: confusion, slurred speech, fainting or a body temperature above 103 degrees. Los Angeles County residents can dial 211 or check the county's emergency site for cooling-center locations and heat alerts. Those steps save lives in the moment. The longer-term arc of the problem — nights that keep getting warmer — is, scientists stress, tied to the broader trajectory of a heating climate. ## Sources - [Climate change rule of thumb: cold things warming faster than warm things](https://www.climate.gov/news-features/blogs/beyond-data/climate-change-rule-thumb-cold-things-warming-faster-warm-things) - [Heat and people with cardiovascular disease](https://www.cdc.gov/heat-health/hcp/clinical-overview/heat-and-people-with-cardiovascular-disease.html) - [Urban heat waves and vulnerable communities in Los Angeles](https://today.usc.edu/urban-heat-waves-los-angeles-vulnerable-communities-usc-research/)