Summer's hot temperatures can make people feel exhausted - but also make them extremely grumpy, a study claims.
Northwestern researchers found that when people are uncomfortably hot, they are less willing to be helpful and are often more moody.
Previous studies show that in addition to having foul moods, soaring temperatures can cause people to be more violent and aggressive, with crime rates peaking during the summer.
Experts believe these negative behaviors stem from exhaustion and dehydration, which puts people in testier moods.
The study was done in three parts and conducted by researchers at Northwestern University in Illinois and Lehigh University in Bethlehem, Pennsylvania. The goal of the study was to examine how heat influenced people to be less helpful.
Researchers first looked at data provided by a Russian retail chain that allowed them to see how environments impacted workers.
Those working in hot temperatures were 50 percent less likely to help customers, actively listen to someone and to make suggestions.
Liuba Belkin, lead author of the study, said in an interview with Quartz: 'To our knowledge, this was the first study to establish the connection between ambient temperature and a reduction of prosocial behavior with data.
'The point of our study is that ambient temperature affects individual states that shape emotional and behavioral reactions, so people help less in an uncomfortable environment, whatever the reason they come up with to justify why they cannot do.'
In the second part of the study, researchers used an online survey where half of the paid participants were asked to recall a time they were uncomfortably hot.
After finishing a set of questions, both groups were asked to help with a second study, but they wouldn't be paid.
Out of those who were asked to recall a time they were hot, only 34 percent agreed to help out with the second survey. This was compared to the other group who weren't asked about a hot day, where 74 percent willingly did the second survey for free.
The last part of the study found people in a hot room were less likely to fill out a questionnaire at 64 percent, compared to people in a cool room at 95 percent.