You’ll be on edge if you don’t get a good night’s sleep, a new paper reports.

Image credits Manfred Richter.

Cutting a few hours of sleep out of your schedule isn’t as good an idea as it may appear. Researchers at the Iowa State University report that missing sleep will make you angrier, leaving you ill-equipped to deal with frustrating situations. This is one of the first studies to provide evidence that sleep loss causes anger.

Get mo’ sleep

“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,” said Zlatan Krizan, paper first author and a professor of psychology at Iowa State.

The link between lack of sleep and a predisposition to foul mood (anger included) has been documented in past research, but a direct cause-effect relationship couldn’t be established. In other words, we knew that the two come together, but not whether one causes the other. In the current study, Krizan and co-author Garrett Hisler, an ISU doctoral student in psychology, tackle this question — and also provides new insight into our ability to adjust to irritating conditions when tired.

Participants were randomly assigned to one of two groups. The control group maintained their normal sleep routine, while the experimental group cut two to four hours each night for two nights. The first group averaged almost seven hours of sleep a night, while the restricted group got about four and a half hours each night. Krizan says this difference is an accurate reflection of the sleep loss we regularly experience in everyday life.

In order to measure anger, the duo had participants visit the lab before and after the sleep manipulation to rate different products. All the while, they were listening to brown noise (similar to the sound of spraying water) or more aversive white noise (similar to a static signal). Krizan says the purpose was to create uncomfortable conditions, which tend to provoke anger.

“In general, anger was substantially higher for those who were sleep restricted,” Krizan said. “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.”

Sleep loss is known to increase negative emotions, such as anxiety and sadness, and decreases positive emotions, such as happiness and enthusiasm. However, the team says they found that sleep loss uniquely impacted anger — it didn’t just result from feeling more negative at the moment. They also tested whether subjective sleepiness (i.e. how sleep-deprived participants felt) explained more intense feelings of anger. They report it accounted for 50% of the effect on anger, which suggests that an individual’s sense of sleepiness may indicate whether they are likely to become angered, Krizan said.

Krizan is also working on a separate study on whether these effects carry over to daily life. It involves 200 participants who were asked to keep a sleep diary for a month — each day, the students recorded their sleep and rated feelings of anger. Initial results show students consistently reported more anger than what was typical for them on days when they got less sleep than usual. Krizan and Hisler are also collecting data to test if sleep loss is a driver of aggressive behavior toward others.

The paper “Sleepy anger: Restricted sleep amplifies angry feelings” has been published in the Journal of Experimental Psychology: General.