A recent study published in the journal Chronobiology International suggests that people who naturally prefer staying up late might be more likely to exhibit sadistic personality traits. The findings provide evidence that night owls tend to derive more pleasure from inflicting harm on others compared to early risers. This relationship helps scientists understand how harmful personality traits might have adapted to specific environments, such as the cover of darkness.
The research was conducted by Heng Li, a scientist at the Center for Linguistic, Literary and Cultural Studies at Sichuan International Studies University in Chongqing, China. Li designed the research to explore how an individual’s biological clock relates to negative, antisocial behavioral tendencies. Specifically, the scientist focused on a biological concept known as chronotype.
Chronotype refers to a person’s natural physical inclination to sleep and be active at certain times of the day. Some people naturally wake up early and feel most alert in the morning, earning them the nickname of morning larks. Others feel most awake and productive late at night, making them night owls.
Previous studies have linked this nighttime preference with dark personality traits, such as narcissism, manipulation, and a lack of empathy. To explain this link, evolutionary scientists often point to the niche-specialization hypothesis. This is a scientific concept suggesting that certain antisocial traits developed to help people thrive in very specific environments.
During the night, there is less light, and fewer people are awake to monitor or judge social behavior. In this darker, quieter environment, individuals who want to break rules or manipulate others face a much lower risk of being caught and punished. The researcher noticed that past studies had largely ignored how this nighttime preference relates to everyday sadism.
Everyday sadism is a personality trait where a person experiences enjoyment or amusement from causing physical or emotional pain to others in normal, daily life. People with high levels of this trait might enjoy bullying others online or playing cruel pranks.
“During the COVID-19 pandemic, I noticed that individuals with a strong preference for late-night activity — often called ‘night owls’ — seemed more likely to disregard public health guidelines,” Li told PsyPost.
“This observation raised an intriguing question: could there be a deeper psychological connection? One possibility is that these individuals might derive a subtle sense of gratification from the challenges or suffering others experience during a crisis. To explore this further, I conducted a study investigating the relationship between chronotypes (our internal biological clocks) and everyday sadism.”
To test this hypothesis, the researcher conducted two separate studies using different methods. In the first study, Li recruited exactly 170 Chinese university students. The sample had an average age of 22 years and a nearly even split between men and women, with participants coming from various geographic regions across China.
These students completed a series of surveys designed to measure their natural sleep habits and personality traits. The participants answered detailed questions about what time they would prefer to wake up if they had no obligations or work schedules to follow. They also rated how alert they generally feel at different times of the day.
Next, the students completed a widely used psychological survey that measures everyday sadism. This survey asked participants to agree or disagree with statements about whether they enjoy hurting others or having fantasies about causing harm. Because people often lie to look good on psychological surveys, participants were guaranteed complete anonymity to encourage honest answers.
The data from this first study showed a clear, measurable link between sleep preferences and personality. The results provided evidence that students who strongly preferred the evening tended to score much higher on the measure of sadistic tendencies. Men generally scored higher in overall sadism than women, but both men and women showed the exact same relationship between being a night owl and having sadistic traits.
While the first study relied entirely on self-reported surveys from university students, the researcher wanted to verify these findings in a broader population using a real-world task. For the second study, Li recruited 214 adult participants from southwestern China. These non-student adults had an average age of about 33 years, and just over half of them were women.
The participants once again filled out the survey assessing their natural sleep and wake preferences. However, instead of answering written questions about sadism, they participated in a physical behavioral experiment in a laboratory setting. The scientist presented the participants with a modified coffee grinder, which was falsely described to them as a machine that crushes living insects.
The participants were shown three live pill bugs, which had been given common Chinese human names to make them seem more relatable and to make the task feel more cruel. The researcher then gave the participants the option to drop the bugs into the machine and grind them to pieces. The machine was actually altered with a hidden barrier so that no bugs were ever harmed, but it produced a loud crunching noise to make participants believe the bugs were being destroyed.
Out of the 214 adult participants, 25 individuals voluntarily chose to act as exterminators and put at least one bug into the machine. When analyzing the results, Li found that a person’s sleep preference significantly predicted their choice to harm the bugs. Participants with a nighttime chronotype were statistically more likely to choose to crush the insects, providing objective evidence that an evening preference is associated with actual sadistic behavior.
While these findings help explain the biology behind dark personalities, the researcher noted a few limitations to keep in mind. The study relies entirely on observational data, meaning it can only show a mathematical relationship between sleep preferences and sadism. It does not prove that staying up late physically causes a person to become sadistic.
It is entirely possible that the relationship works the other way around, or that another unmeasured biological factor influences both traits at the same time. Future studies might use different methods to track human behavior over a longer period. For example, scientists could temporarily alter a person’s sleep schedule in a laboratory to see if changing their biological rhythm directly impacts their aggressive behaviors.
Additionally, the current research measured sleep preferences by asking people to report their own habits on a questionnaire. People are not always accurate when remembering or estimating their sleep schedules. Future investigations might use wearable technology, like fitness trackers, to objectively record exact sleep and wake times over several weeks.
Understanding this biological link offers practical applications for monitoring harmful behavior in society. For instance, knowing that sadistic tendencies peak during nighttime hours could help internet platforms better moderate cyberbullying when it is most likely to occur. It might also help law enforcement agencies understand when manipulative or cruel behaviors are most prevalent in public spaces.
“It is crucial, however, that the general public does not over-interpret these findings or harbor bias against those who prefer the night. Our results represent a statistical correlation, not a universal rule,” Li noted. “Human personality is incredibly complex, shaped by a vast array of genetic and environmental factors. Simply being a nighttime person does not define one’s character; rather, our research suggests that the quiet, unsupervised hours of the night may simply offer a unique ‘ecological niche’ where certain dark personality traits are more likely to surface.”
The study, “Night owls and dark hearts: The link between chronotype and sadistic tendencies,” was authored by Heng Li.