The summer of 2026 has been setting records for high temperatures and heat-related deaths in Europe and the U.S. Older adults and those with chronic health problems are affected most, but everyone is affected to some degree. Considerable research has been done to examine how high temperature and humidity affect the quality of our sleep. This is a good time to summarize what we know.
First, let’s look at what happens to our body temperature during a normal night’s sleep. Regulated by the circadian cycle, our core temperature drops slightly at sleep onset and continues to drop, reaching its lowest point after about six hours. Many of us can relate to going to sleep covered by only a sheet and waking up later to grab a blanket. The drop is caused by the metabolic rate slowing down and producing less heat. After the low point, body temperature begins to rise as part of the circadian process leading to waking up. Controlling the ambient temperature in the bedroom is critical for supporting good sleep. Because air conditioning in houses and apartments is less available in northern European countries than in the U.S., cooling to a comfortable temperature there is more challenging.
How Humidity and Airflow Affect Sleep Quality
High humidity mixed with heat is especially uncomfortable and we are all familiar with the heat index or what is sometimes called the “real feel.” A high-quality whole-house HVAC system cools by drawing heat and moisture out of the house. Good airflow and ventilation are also necessary for optimal sleep. Window units are less effective and window or ceiling fans even less so. Portable dehumidifiers are cost-efficient and effective in homes without good HVAC systems.
When researchers have manipulated temperature and humidity and measured sleep (time to fall asleep, number and length of nighttime awakenings, REM and slow-wave periods, and overall duration), they have concluded that ambient bedroom temperatures between 62 and 82 degrees Fahrenheit and relative humidity between 40% and 60% are most conducive to good sleep. Of course, there are individual differences that can sometimes pose challenges, as is often the case when two or more people share bedrooms, beds, and bedding.
Why Better Cooling Is Good for Your Health
Children are more susceptible to ill effects of high temperature and humidity than adults because of their higher ratio of body surface to mass. In studies done in our lab at Auburn University, we have found that in families with lower financial resources, good control of nighttime bedroom temperature is often not available, and children’s sleep is poorer.
Since we know that poor sleep can cause or exacerbate many health problems, high heat and humidity can affect health directly and also indirectly through poor sleep. For those who have the means to do so, installing new or upgrading existing cooling and dehumidifying systems is a good investment not only for comfort but also for health.
