Temperature has a big influence on sleep quality. Our circadian rhythm directs our blood vessels to dilate when the body is preparing for sleep, leading to a drop in body temperature which is maintained through the night. As morning arrives, our body temperature begins to rise, acting as a signal that it’s time to wake up.
It makes sense, therefore, that warm temperatures at night can hinder sleep — and this prompted researchers to speculate whether global warming might disrupt sleep in the future.
A 2017 research article published in the journal Science Advances set out to answer four key questions:
- Do unusually high nighttime temperatures harm sleep quality?
- Do the effects of nighttime temperatures on sleep vary by season?
- Are the effects worse for those least able to cope with unusual nighttime heat?
- Could nighttime warming caused by climate change increase the rate of insufficient sleep in the future?
Researchers used data from the Centers for Disease Control and Prevention Behavioral Risk Factor Surveillance Survey (BRFSS). Data was collected between 2002 and 2011 from 765,000 American residents.
To measure sleep quality, researchers collected responses from the question, “During the past 30 days, for about how many days have you felt you did not get enough rest or sleep?”
Individual responses were combined with weather and climate data from each participant’s nearest city. When analyzing the data, researchers took steps to prevent characteristics of individual cities such as noise pollution levels and air conditioning use from interfering with the estimated effect of nighttime temperature on sleep.
The effect of nighttime temperature on sleep
Researchers found that as unusually warm temperatures became more common, so did nights of insufficient sleep. In fact, the study revealed that a change in temperature of just 1°C (1.8°F) led to an increase of roughly three nights of insufficient sleep for every 100 individuals per month.
If these findings were replicated to account for the entire population of the United States, an increase of 1°C across the country would lead to almost nine million additional nights of insufficient sleep per month (or roughly 110 million extra nights of insufficient sleep per year).
Seasonality and coping ability affects sleep, too
The study found that the effect of above-average temperatures during the summer was almost three times that observed during any other season.
The data also revealed that above-average temperatures had a more than three-fold effect on the sleep of those with incomes under $50,000 compared to those with incomes over $50,000. Furthermore, the sleep of those over 65 was nearly twice as likely to be affected by higher temperatures compared to those under 65.
How climate change could harm our sleep
To determine whether an increase in nighttime temperatures due to climate change may have a negative impact on sleep, researchers calculated projected nighttime temperatures anomalies for 2050 and 2099 using data from NASA projections. They found that climate change may cause approximately six additional nights of insufficient sleep for every 100 individuals by 2050, rising to more than approximately 14 nights by 2099.
The study also found that those in the western and northern United States may see the biggest increase in temperature-related sleep deprivation since these areas are projected to see the most significant changes in nighttime temperatures in the future.
Although this study found that rising temperatures do indeed have a negative impact on sleep, there are a few caveats to bear in mind. First, this study used self-reported data on sleep duration rather than objective measures such as actigraphy or polysomnography.
The study also failed to consider other factors that can influence sleep duration such as comorbid health conditions, stress levels, and even occupations.
Finally, it’s worth bearing in mind that the occasional temperature anomalies used in this study may have a bigger impact on sleep than a gradual and consistent increase in nighttime temperatures over time since our bodies may be better able to adapt to a slow, consistently warming climate.
With that being said, since we already know that temperature plays a big role in the regulation of our sleep-wake cycle, it makes sense that rising temperatures could have a negative effect on sleep.
It also makes sense that these negative effects are likely to have a bigger impact on the elderly and the poor since the bodies of older adults are less tolerant to temperature changes and the poor may be less able to take advantage of air conditioning.
This study should be taken as further evidence of the important role temperature plays in our overall sleep health. It also serves as the first piece of evidence to suggest that a warming climate could have a negative effect on sleep in the future.