Losing just two hours of sleep a night makes people angrier, research reveals.
The study is among the first to prove a direct link with lack of shuteye, experts say.
Previous research shows sleep loss increases negative emotions, such as anxiety and sadness, and decreases positive emotions, including happiness and enthusiasm.
But researchers at Iowa State University found losing sleep raised anger directly -- not just as a result of feeling more negative. Their findings were published in the Journal of Experimental Psychology.
Participants were split into two groups. One kept to their normal sleep routine while the other had theirs cut by two to four hours each night for two nights.
The first group had almost seven hours of sleep a night, while the restricted group got about four and a half hours.
The difference was designed to reflect what people regularly experience in everyday life.
Both groups were then asked to rate different products while listening to irritating noise. The researchers said this was meant to create uncomfortable conditions and provoke feelings of anger.
Study leader professor Zlatan Krizan said: "In general, anger was substantially higher for those who were sleep restricted."
"We manipulated how annoying the noise was during the task and, as expected, people reported more anger when the noise was more unpleasant. When sleep was restricted, people reported even more anger, regardless of the noise."