It may sound counterintuitive, but the lazy days of summer can be the worst for sleep.
To dig into the science of how summer’s excessive heat affects our sleep, we’ve teamed up with SleepScore Labs, our dream team and partners in sleep data and science, to uncover the ideal temperature for supporting sleep.
We’ll also share tips to help you enjoy cool, comfortable sleep on hot summer nights.
How Does the Body’s Temperature Change When We Sleep?
Just like the fluctuations in outdoor weather, your body temperature changes over the course of the day. These changes help your body to align its circadian rhythms (the body’s internal clock).
During the day, your exposure to sunlight signals to your brain that it’s a time to be awake, setting the circadian clock accordingly. This prompts the body’s natural temperature to reach its baseline, which is around 98.2 degrees Fahrenheit (though the range can vary by person, from 97.7 to around 98.6).
As the sun goes down, this change stimulates the release of melatonin, which signals feelings of tiredness and even a dip in core temperature to optimize your body for sleep. You can think of it as your brain setting its thermostat to rest mode, which is around 1 to 2 degrees lower than your daytime temperature.
While this may not sound like much, if your temperature were to increase by those 2 degrees, it could be categorized as a low-grade fever!
Ways the Body Cools Down When It’s Hot
Body temperature is lowered when heat is released through your skin and your breath. Blood vessels transport heat through your body, and when it’s warm, the vessels — particularly the ones near the skin — widen (or vasodilate) to allow for more heat exchange, expelling body heat.
The second mechanism is evaporative heat loss, also known as sweating. “When the body has a hard time losing heat this way, it can increase the heat loss by opening up extra shunting vessels near the skin and/or by removing even more heat via sweating,” says Roy Raymann, Ph.D., principal sleep scientist and innovator at SleepScore Labs.
Opening shunting vessels allows for more rapid heat exchange with the air. These vessels are mainly located in hands, ears, and fingertips, which is why they're the first to turn red when you're warm.
According to Raymann, hot days make it harder to reach lower body temperatures at night, because the body has to do more to lose heat, including through sweating.
As the body is busy managing and adjusting its temperature – which requires more effort when temperatures soar – there is a higher likelihood that sleep will be disturbed because both additional methods of allowing the body to cool are associated with sympathetic nervous system activity, your body’s fight-or-flight response.
How Does Sleep Change During Hot Weather?
Exposure to extremely warm nighttime temperatures, like those common during balmy summers, can wreak havoc on our sleep. For example, studies have shown that sleeping in an environment between 87-to-100 degrees Fahrenheit both interrupts sleep and is associated with a significant reduction in important sleep stages such as slow-wave sleep and rapid-eye-movement (REM) sleep.
A SleepScore Labs analysis of over 3.75 million nights published in the journal SLEEP revealed that higher bedroom temperatures were associated with more time needed to fall asleep, more nighttime awakenings, and less total sleep.
The Impact of Extreme Heat and Humidity on Sleep Quality
It’s clear that excessive overnight temperatures can significantly impair sleep. But hot weather can mean different things to different regions, so we wanted to explore how sleep quality changes during the hottest months in places where temperatures are highest year-round.
Fascinatingly, a study on people living in the Sahel regions of Africa reveal, at least in part, some positive effects: Long-term heat acclimatization and exposure to extreme daytime temperatures during the summer (between 95-99 degrees Fahrenheit) actually increase deep sleep when participants sleep in an air-conditioned environment at night. There was still sleep disruption, including increased nighttime wakefulness and lighter sleep stages (NREM 1), but the extreme high daytime heat in these regions could have a primary, selective effect on slow-wave sleep.
Though striking, the results may not hold for more humid locations. Researchers suggest that humidity makes it difficult for us to expel heat, as it increases thermal load and impairs evaporation.
“It remains unclear how these findings can be generalized to other parts of the world that experience less consistently hot temperatures and are drastically more humid,” explains SleepScore Labs’ applied sleep scientist, Elie Gottlieb, Ph.D.
For example, studies in Japan show a reduction in total sleep time and sleep efficiency during humid summer months compared to fall or winter. Another article published in the journal SLEEP found that high levels of humid heat increased nighttime awakenings and reduced deep sleep.
Hot weather can also lead to secondary environmental factors, including natural disasters, that disturb sleep. For example, a 2013 study found that Australian residents whose households were exposed to summer floods and wildfires were twice as likely to report poor sleep quality.
In 2003 an anticyclone over Europe caused a devastating summer heat wave that claimed over 30,000 lives, nearly 15,000 in France alone.
Not All Heat Is Bad for Sleep
Though excessive heat can worsen sleep, some heat can help improve it.
According to Gottlieb, “Generally, exposure to extreme heat during the night leads to sleep disruption, but some studies show that mild heating before bed may actually have the opposite effect.”
For example, exposure to short-term and mild heating before sleep has been shown to increase slow-wave sleep. These effects have been observed in controlled studies where participants took a warm bath or spent time in a sauna.
The timing of this mild heat before sleep is important. A recent study found that warm baths taken two to three hours before bed significantly reduced the time taken to fall asleep — by around 20%. However, more research is needed to determine whether showers or other forms of passive body heating can also help improve sleep, and whether hot summer days and nights can impact these effects.
The Ideal Temperature for Sleep
The recommended bedroom temperature range for sleep is typically between 60 and 72 degrees Fahrenheit, with 65 degrees considered the sweet spot for most people, according to Raymann.
To achieve a lower body temperature more easily, your bedroom temperature should be significantly lower than your body temperature without feeling too cold to you or your bed partner.
If your bedding includes multiple layers of comforters or duvets, you might prefer a room temperature setting closer to 60 degrees.
The opposite is also true: If your preferred duvet is light or if you sleep without covers, you might prefer a warmer setting.
“In the end, it’s all about feeling comfortable in your bed,” says Raymann. “The room temperature should be such that your body has no trouble keeping its temperature around its nocturnal setpoint.”
Does the Recommended Temperature Change as People Age?
As we age, we have a diminished capability to sense the body’s temperature correctly, as well as to generate warmth and retain it. This can mean setting the thermostat a bit higher or adding an extra blanket to the bed.
The key is that the temperature should be comfortable for you personally, and that range can change over time.
Tips for Keeping Cool During Sleep
To sleep soundly, the core temperature should be cooler than daytime, but the skin temperature should be comfortably warm.
The best strategy to help your body get rid of heat during a summer heat wave is to support cooling down before going to bed. A cool or lukewarm bath or shower can help with that, suggests Raymann.
For most people, the temperature in the bedroom at night should not exceed 72 degrees Fahrenheit, and it should be even lower for people that sleep best in a cool bedroom.
Ideally, your bedroom climate control would kick in well before bedtime to help your room reach the target temperature, especially if you still have an air-conditioning unit that is relatively noisy.
Think about unmaking your bed during that pre-cooling period, so the mattress and bedding can cool down as well. And finally, you might want to switch to lighter, more breathable bedding.
Should you need it, there are a range of cooling products, including pillows, mattresses and even cooling pads to help compensate for high heat, so that you can sleep cool and feel great.
READ NEXT: Cooling Tips to Help With Night Sweats and Cold Flashes
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