Too Hot to Handle? Decoding How Humidity Affects Your Sleep

‍Reviewed by Sahil Chopra, MD.

Research by Savit Malhotra and Quynh Theresa Do.

Introduction 

When checking the weather forecast, most of us immediately look at the temperature. However, have you ever noticed that a 26°C (80°F) day can feel perfectly comfortable one day and absolutely miserable the next? The difference is often humidity. High humidity makes it harder for your body to cool itself, while very dry air can irritate your nose, throat, and eyes.^[1] Both can interfere with your ability to fall asleep and stay asleep.^[2] As climate change contributes to more frequent and more intense heat waves, warmer nights are becoming a recognized threat to sleep worldwide. Understanding how the bedroom environment affects your sleep can help you wake up feeling more refreshed.

What is Humidity?

Humidity refers to the amount of water vapor in the air. There are two types of humidity measurements: absolute and relative. Absolute humidity is the actual amount of water vapor, or moisture, in the air. Meteorologists typically report relative humidity (RH), which compares the current moisture in the air to the maximum moisture the air can hold at that temperature.^[3] This number is expressed as a percentage. For example, an RH of 50% means the air is holding half of the moisture it is capable of holding at that temperature. An important point is that RH depends on temperature. Because warm air possesses a higher moisture capacity than cold air, a constant amount of absolute or specific humidity translates to differing relative percentages depending on temperature. Therefore, cooler air will exhibit a higher RH, whereas warmer air will register a lower RH. This is why a summer afternoon with 50% RH can contain significantly more moisture than a cool autumn day with the same RH reading. In other words, two days with identical RH can feel very different because the actual amount of water vapor in the air is different. RH plays an important role in meteorology, the evaluation of thermal comfort, and our subjective experience of heat. Because it is influenced by temperature and the air's moisture capacity, RH is valuable for understanding skin evaporation rates, managing cooling systems, and predicting the weather.^[4]

As the planet warms, the atmosphere's capacity to hold water vapor also increases. Warmer air can hold roughly 7% more water vapor for every 1°C (1.8°F) of warming. This physical relationship is known as Clausius-Clapeyron scaling.^[5] Importantly, this means that the total amount of moisture in the air is rising. Observations show that the atmosphere now contains substantially more water vapor than it did several decades ago. This increase has intensified the global water cycle, contributing to heavier rainfall and increasing the likelihood of extreme precipitation events.^[6] While the total amount of atmospheric moisture has increased, RH has not risen uniformly. Over many continental regions, RH has remained stable or even declined because land surfaces are warming faster than moisture can be replenished. In contrast, coastal and tropical regions often remain highly humid because nearby oceans provide a continuous source of atmospheric moisture.^[7]

How Humidity Impacts Sleep

Sleep is tightly linked to body temperature, as we discussed in our previous blog on temperature and circadian rhythms. In the evening, rising melatonin triggers blood flow to the hands and feet, which allows the body to dump heat and nudges core temperature downward. Sleep typically begins on the falling slope of that curve.^[8] When carefully measured, falling asleep itself lowers core temperature by 0.3°C (0.5°F) beyond the normal nightly circadian decline.^[9] People whose core temperature falls more slowly before bed tend to have poorer sleep and higher overnight heart rates.^[10] Humid heat interferes directly with this cooling process. In a classic laboratory study, healthy young men slept in four combinations of temperature and humidity. At 35°C (95°F) with 75% RH, they spent more time awake, had lower sleep efficiency, and their rectal temperature stayed elevated all night. Additionally, compared to at 29°C (84.2°F) with 75% RH, the participants had significantly less deep (slow-wave) sleep as well as less REM sleep.^[11] A follow-up study showed that humid heat is especially disruptive when it hits later in the night, and that air conditioning during the early hours of sleep helps protect both sleep and thermoregulation.^[12]

Humidity also influences the air you breathe. In a study of more than 5,000 adults undergoing overnight sleep studies, higher outdoor humidity was associated with a higher apnea-hypopnea index (AHI), more time spent sleeping on the back, and more arousals. Interestingly, the researchers found that higher outdoor humidity was associated with less snoring.^[13] AHI, as we have discussed extensively in previous blogs and our recent publication, is a measure of sleep-disordered breathing. In a larger analysis from the same sleep center, humidity, temperature, and fine particle pollution (PM_2.5) were all linked to changes in sleep stages and arousals, and the effects were intertwined. Air pollution partly explained the humidity and temperature associations, especially in the colder months.^[14] In other words, humidity, as seen through these studies, rarely acts alone. Indoor humidity also drives allergens that impact sleep. Dust mite populations are minimized when RH stays below about 50%, and most molds cannot grow unless humidity exceeds about 60%.^[15] Keeping humidity in check is a simple way to reduce both.

While high humidity dominates summer conversations, air that is too dry can interfere with sleep and comfort as well. Low indoor humidity is associated with the following symptoms^[16,17]: 

• Dry, tired, irritated eyes
• Dry mouth
• Dore throat
• Nasal dryness and crusting
• Itchy skin
• Less efficient clearance of mucus from the airways
• More frequent upper-airway symptoms 

Notably, one analysis found that lower humidity and temperature were associated with reduced deep sleep and more arousals, particularly during colder months when indoor heating dries the air.^[18]

Recommendations for the ideal range differ depending on goals. Guidance aimed at mold and dust-mite control commonly targets 30–50% RH, staying below 60% RH. The indoor air-quality literature more often favors a 40–60% “middle zone” RH range, as best for eye and airway health and for lowering the survival of airborne viruses.^[16,17] A Cochrane review concluded there is no firm consensus on a single optimal RH number.^[18] Anywhere in the 30–60% band is reasonable. The extremes are what to avoid.

What To Do

When considering how to manage your sleep environment, several recommendations can be helpful. More tips can be found in our article on bedroom environments as well.

• Aim for roughly 30-50% indoor relative humidity (some experts favor 40–60%). Below 50% suppresses dust mites; below 60% suppresses mold.^[19]
• Keep your bedroom cool, but don't obsess over one number. The often-quoted 60–67°F (15.6–19.4°C) figure comes from a National Sleep Foundation recommendation rather than from clinical trials. Field studies of people actually sleeping with bedding, which creates its own warm microclimate, find comfortable sleeping temperatures closer to 75–79°F (24–26°C). Cool enough that you're not sweating is the practical target.^[20,21]
• Measure it. A digital hygrometer is inexpensive and takes the guesswork out.
• Use a dehumidifier if humidity runs above 60%.
• Run your air conditioner, which cools and dehumidifies at the same time — and prioritize the first half of the night, when heat exposure appears most disruptive. One small randomized crossover trial found that cooling methods that lowered humidity and air movement at the same room temperature improved sleep efficiency and shortened time to fall asleep.^[22]
• Ventilate. Better airflow lowers CO₂ and heat, both linked to lighter sleep.^[23]
• Choose breathable bedding such as cotton or linen.
• Stay hydrated, especially during heat waves.

Conclusion

Humidity may be easy to overlook, but it can meaningfully shape how comfortable and restorative your sleep feels. When the air around us is too humid, it can prevent our bodies from releasing heat and also increase nighttime awakenings. However, when the air is not humid enough, we may experience irritation in our eyes, skin, throat, and airways. While there is no “perfect humidity level” for everyone, keeping the bedroom within the range of 30% to 60% can help mitigate the effects of either extreme side of humidity. Keeping a hygrometer, along with air conditioning, ventilation, a humidifier/dehumidifier when needed, and breathable bedding can help make managing your sleep environment easier. As warmer and more moisture-filled nights become increasingly common, paying attention to both temperature and humidity may be an important part of protecting your sleep. If you’re currently experiencing the effects of high humidity, or are curious to learn more about how humidity affects your sleep, we encourage you to reach out to a professional to explore more. The team at Empower Sleep is always available to help you with your sleep needs!