During a heatwave, nights can stay hot because buildings, pavement, and the air around them retain the day’s heat. That leaves your home and body with less time to cool down. Reduce daytime sun entering the bedroom, ventilate only when outdoor air is cooler and safe to bring inside, and use air conditioning or a cooler location if indoor heat becomes unsafe. Fans can help sweat evaporate, but they do not lower room temperature and have important safety limits.
Why does a heatwave stay hot after sunset?
A heatwave can mean several unusually hot days and nights in a row. Walls, roofs, roads, and other hard surfaces absorb heat during the day and release it slowly. If the outdoor air also stays warm, opening a window may bring in little relief. The home and the body get less respite between hot days. The World Health Organization (WHO) warns that extended high temperatures by day and night create cumulative stress on the body (WHO heat and health fact sheet).
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Why cities can feel hotter at night
Urban neighborhoods can hold more heat than nearby outlying areas. Roads, sidewalks, and buildings absorb and retain sunlight, while limited vegetation and built forms that restrict airflow can reduce cooling. The U.S. Environmental Protection Agency (EPA) reports that U.S. urban areas are about 1–7°F warmer during the day and about 2–5°F warmer at night than outlying areas. Those are U.S. comparisons, not a prediction for every neighborhood or country (EPA: Heat Islands and the Heat Island Effect).
How nighttime heat can affect sleep
Hot nights are associated with shorter sleep in population research, often because people take longer to fall asleep. A 2020 study by Kelton Minor, Andreas Bjerre-Nielsen, Sigga Svala Jonasdottir, Sune Lehmann, and Nick Obradovich analyzed more than 7 million nighttime sleep records from 68 countries alongside local weather data from 2015–2017. It found that rising nighttime temperatures shortened sleep duration within individuals, primarily through later sleep onset; larger effects were reported among older adults and residents of lower-income countries. This is an observational population finding, not a guarantee about how any one person will sleep (“Ambient heat and human sleep,” One Earth).
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Cool the bedroom in the right order
1. Keep daytime heat out
When outdoor air is hotter than indoor air, keep windows closed. Cover windows that receive direct morning or afternoon sun; outdoor awnings and louvers can block more incoming heat than indoor shades, according to the EPA. Sun-facing window shades or blinds can also reduce direct solar gain (EPA heat island cooling strategies).
2. Ventilate only when conditions favor it
After dark, compare the outdoor air with the room. Open windows only when outside is cooler than inside and it is safe to do so. If outdoor air is hotter, keep the windows closed. Consider local air quality as well: smoke, pollution, or other unsafe conditions may make bringing outdoor air inside inappropriate. WHO and EPA both advise limiting ventilation to circumstances when it can help cool the home (WHO heat and health fact sheet; EPA heat island cooling strategies).
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3. Use air conditioning or go somewhere cooler when needed
Air conditioning, or time in an air-conditioned public building, is a more direct way to lower indoor air temperature than a fan. If you cannot keep the home cool, WHO advises moving to the coolest room and spending time in an air-conditioned public building. If you do not have air conditioning, plan ahead for a place you can go during dangerous indoor heat (WHO heat and health fact sheet; EPA heat island cooling strategies).
WHO gives indoor targets of below 32°C during the day and 24°C at night, especially for infants, adults over 60, and people with chronic conditions. These are WHO guidance, not a universal sleep prescription; a room thermometer can help you check indoor conditions (WHO heat and health fact sheet).
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4. Treat fans as airflow, not air conditioning
A bedside or room fan moves air and can help sweat evaporate; it does not cool the air in the room. Official thresholds differ, so follow the guidance that applies where you live:
- U.S. CDC: Use indoor fans only when the temperature is below 90°F. At higher temperatures, a fan may increase body temperature (CDC: Heat and Cold Stress).
- U.S. EPA: Do not use an electric fan in a closed room or when the temperature is in the mid-90s °F or higher (EPA heat island cooling strategies).
- WHO: Use electric fans only below 40°C; fans may not prevent heat-related illness when temperatures exceed 35°C (WHO heat and health fact sheet).
These agencies do not give one identical cutoff. Do not rely on a fan instead of air conditioning or a cooler place when the room is dangerously hot.
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Cool your body for sleep
- Choose light, loose clothing and lightweight bed linens.
- Take a cool shower or bath before bed.
- Use damp cloths, a light spray of water, or light wet clothing to help cool your skin.
- Drink fluids regularly. Individual needs vary, particularly for people with medical conditions, so follow any fluid guidance from your clinician rather than relying on a general target.
These measures can make you more comfortable, but they do not replace effective cooling when indoor temperatures are unsafe (WHO heat and health fact sheet).
Recognize heat illness and get help
Heat can become a medical emergency. CDC lists heavy sweating, muscle cramps, shortness of breath, dizziness, headache, weakness, and nausea among symptoms of heat-related illness. If heatstroke is suspected, WHO advises getting emergency medical help immediately and actively cooling the person while help is on the way (CDC: Heat and Cold Stress; WHO heat and health fact sheet).
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Infants and young children, adults 65 and older, pregnant people, and people with heart or other chronic conditions can be more vulnerable to heat. Risk can also rise with outdoor work or exercise. Check on people who may have difficulty cooling themselves or reaching a cooler place.
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