Losing just a couple hours of sleep at night makes you angrier, especially in frustrating situations, a study has found.
Other studies have shown a link between sleep and anger, but questions remained about whether sleep loss was to blame or if anger was responsible for disrupted sleep, said Zlatan Krizan, a professor at Iowa State University in the US. The research answers those questions and provides new insight on our ability to adjust to irritating conditions when tired.
"Despite typical tendencies to get somewhat used to irritating conditions – an uncomfortable shirt or a barking dog – sleep-restricted individuals actually showed a trend toward increased anger and distress, essentially reversing their ability to adapt to frustrating conditions over time. No one has shown this before," Krizan said.
Study participants were randomly split into two groups: one maintained their normal sleep routine and the second restricted their sleep by two to four hours each night for two nights.
Those who maintained averaged almost seven hours of sleep a night, while the restricted group got about four and a half hours each night.
The difference reflects sleep loss we regularly experience in everyday life, Krizan said.
To measure anger, researchers had participants come to the lab – before and after sleep manipulation – to rate different products while listening to brown noise or white noise.
Krizan said the purpose was to create uncomfortable conditions, which tend to provoke anger.
"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," said Krizan.
"When sleep was restricted, people reported even more anger, regardless of the noise," he said.
It is well established that sleep loss increases negative emotions, such as anxiety and sadness, and decreases positive emotions, such as happiness and enthusiasm, Krizan said.
Researchers measured these effects to more generally understand the relationship between sleep, anger and emotions. They also tested whether subjective sleepiness explained more intense feelings of anger. Sleepiness accounted for 50 per cent of experimental effect of sleep restriction on anger, suggesting individuals' sense of sleepiness may point to whether they are likely to become angered.