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Studies suggest hot weather can leave people feeling tired, irritable and stressed.

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Got the summertime blues? Taking refuge from the heat may help.

Multiple studies over the past few decades have indicated that heat exposure can negatively affect people's moods. Hot weather has been linked to increased aggression, violence and higher rates of suicide – and, some caution, global warming may make matters worse.

A new U.S.-led study lends support to this research, showing that high temperatures reduce people's emotional well-being. When exposed to warmer weather, survey respondents reported greater fatigue and increased negative emotions, including sadness, stress and anger, and reduced positive emotions, such as joy and happiness.

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"We see the strongest effects of heat exposure on respondents saying that they lacked energy or felt tired during the day, as well that they reported increased stress and anger," says lead author Clemens Noelke, research scientist at the Heller School for Social Policy and Management at Brandeis University in Massachusetts.

The study, published in the journal Environmental Research, relied on survey results from nearly 1.9 million Americans, from 2008 to 2013. The researchers analyzed these responses along with temperature data for the days participants were interviewed and their self-reported locations. They found negative effects on emotional well-being were especially significant when average daily temperatures rose above 21 degrees Celsius.

For those of us who have endured this summer's sweltering heat waves, 21 degrees Celsius may seem downright balmy.

But Noelke explains the researchers were looking at average 24-hour temperatures, which include night-time temperatures. "That means the maximum could be considerably above that during the day," he says.

Noelke offers a few possible reasons why heat makes people feel awful. Direct heat exposure is intrinsically unpleasant, and for many people, especially those who work outside or who either have limited or no air conditioning, it can be hard to avoid, no matter how much they try, he says. Extreme outdoor temperatures may also force people to stay indoors for extended periods, which can affect their emotional well-being, the study notes.

And Noelke adds there seem to be heat-responsive psychological mechanisms that increase people's anger and arousal, though he says it's unclear to him how they work.

Heat may also simply disrupt people's sleep. "People are a little more tired, a little more irritable, a little more likely to be stressed, and that's also consistent with our findings," he says.

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Noelke points out that he and his team did not find any differences between how people in southern areas, where temperatures are frequently high in summer, respond to heat in comparison with those living in areas with milder summers.

So, for instance, people in Texas, Arizona or Florida were just as unhappy in response to a very hot day as those in Michigan or New York. This seems to suggest that even when people have more exposure to heat, it doesn't necessarily make them any better at adapting to it, Noelke says.

"That's of course a bit disconcerting," he says, since extreme temperatures are projected to increase because of global warming. As high temperatures become the norm, he says, his findings suggest "daily happiness will be affected … and will decline."

But if you can't avoid the heat, it may help to imagine yourself some place cold.

At Ben-Gurion University of the Negev in Israel, Idit Shalev, an assistant professor of psychology in education, and her fellow researchers found in a recent study that exposing participants to cold improved their cognitive control, or self-control.

In one experiment, they asked 87 participants to touch either a cool, room-temperature or warm therapeutic pad before conducting a computer-based test of their cognitive control. Those who were asked to touch the cool pad performed better on the test.

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But in a second experiment, the researchers found that even having participants look at photos of icy landscapes and imagine themselves being there produced similar effects.

"Coldness is an indicator for alertness and self-control," Shalev says, noting that cold temperatures may encourage people to be more cautious. By contrast, cues such as heat and thirst may make them conserve their resources, so they are unable to invest in self-control, she explains.

As her research suggests, Shalev says certain cues seem to rely on perception.

"You don't need to be really hungry or really cold or really under pain," she says. "It's enough that you have some kind of perception that you're just like in a real state of hunger or pain or cold temperature, and it activates the same mechanism."

Still, she cautions that more research is needed before this work can be applied to real life. Trying to improve one's self-control is a complicated and difficult endeavour, she says, and cool temperatures can only do so much.

"The environment can perhaps support your motivation, but the environment is not, you know, a substitute for your motivation," she says. "It's some kind of … facilitator, but not more than that."

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At any rate, if the hot, muggy weather has been getting you down, you'll get a break soon enough. Winter is less than four months away.

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