A lack of quality sleep can cause aggressive behavior, according to recent longitudinal findings published in the journal Biological Psychology. Brain imaging data revealed that the effect may be related to reduced activity in the prefrontal cortex and increased activity in the limbic regions.
Good sleep is paramount to the healthy functioning of our brains and bodies. Studies have shown that a lack of quality sleep can hinder our ability to regulate our thoughts and emotions and inflict consequences on our behavior. One of these consequences might be increased aggression.
While multiple studies have indicated a link between poor sleep and aggressive behavior, the direction of this relationship remains unclear — does poor sleep actually cause aggressive behavior? Study authors Haobo Zhang and Xu Lei conducted a longitudinal study to attempt to answer this question. Through neuroimaging data, they also explored the potential brain mechanism responsible for the relationship between sleep and aggression.
“As sleep plays an important role in the physical and mental health of individuals, we thought to uncover the causal relationship and mechanisms between sleep quality and aggressive behaviour in order to raise public awareness of the importance of sleep,” said Lei, a professor and director of the Sleep and NeuroImage Center at Southwest University in China.
Zhang and Lei obtained data from the Behavioral Brain Research Project of Chinese Personality (BBP), an ongoing study of undergraduate students from Chongqing, China. They focused on data collected from two time points separated by two years. For the current analysis, the sample consisted of around 450 students between the ages of 16 and 26 years old.
At both time points, the participants completed an assessment of their subjective sleep quality in the past month, and a measure of aggression that included the sub-dimensions of hostility, physical aggression, impulsiveness, and anger. Students also underwent functional magnetic resonance imaging (fMRI) to measure their brain activity.
To study the relationship between students’ subjective sleep quality and aggression across time, the researchers used a statistical method called cross-lagged panel analysis. This analysis revealed that sleep quality at Time 1 had a significant effect on aggression at Time 2. By contrast, aggression did not have a significant impact on sleep quality.
“Some researchers have suggested that high levels of aggressive behaviour may also contribute to poor sleep, but our findings do not support such a view,” Lei told PsyPost. “This seems to suggest that the physiological effects of aggressive behaviour are temporary, which should be examined in future studies.”
Importantly, these findings offer tentative evidence of a causal relationship, whereby poor sleep causes increased aggression. To better understand this relationship, the researchers tested the associations between sleep quality and each of the four sub-dimensions of aggression. This revealed that poorer sleep quality was only a significant predictor of increased hostility.
“Sleep is extremely important to humans and poor sleep can increase hostility in individuals, which can damage their interpersonal relationships and have negative consequences for interpersonal interactions,” Lei told PsyPost. “It is therefore important that people make a conscious effort to get enough and good quality sleep.”
The researchers also compared the students’ sleep and aggression scores to their spontaneous brain activity, as measured via their resting-state fMRI activity. These results revealed that poorer sleep quality and increased aggression were tied to weaker activity in certain brain areas, namely in the limbic or frontal regions.
The authors say this may suggest that lower sleep quality led to deficits in emotional cognition — the ability to correctly interpret the emotions of others. The results also revealed that poorer sleep quality and higher aggression were linked to stronger activity in the left and right dorsolateral prefrontal cortex, a brain area implicated in emotion regulation.
There are several proposed explanations for why poor sleep might increase aggression. Since worse sleep only predicted increased hostility and not the other dimensions of aggression, the authors say their findings align best with the General Aggression Model’s cognitive pathway. This interpretation suggests that poor sleep makes people more likely to interpret the behavior of others in a negative light. This greater tendency to attribute someone’s behavior as hostile then encourages aggressive behavior.
A notable limitation was that the study used self-report measures of both sleep quality and aggression. Nevertheless, the study adds to the current research by revealing evidence of a causal relationship from sleep quality to aggression. The findings further suggest that poor sleep may promote aggression by impacting emotional cognition.
The study, “Effect of subjective sleep quality on aggression: A two-year longitudinal and fMRI pilot study”, was authored by Haobo Zhang and Xu Lei.
