New Study Reveals Surprising Effect of Drinking Coffee at Night

Nighttime caffeine increased impulsive behavior in fruit flies, with stronger effects in females and potential implications for night-shift workers.

If you often drink coffee late at night to stay awake, new research suggests that habit may have unexpected effects. Scientists at the University of Texas at El Paso report that consuming caffeine at night may increase impulsive behavior, which could encourage riskier actions.

The research, published in iScience, investigated how caffeine consumed at night influences inhibition and impulsivity in fruit flies. The study was led by Erick Saldes, Ph.D., Paul Sabandal, Ph.D., and Kyung-An Han, Ph.D. According to Han, the fruit fly species used in the experiments, Drosophila melanogaster, is widely used in behavioral research because its genetic and neural systems share important similarities with those of humans.

“Caffeine is the most widely consumed psychoactive substance in the world, with about 85% of adults in the U.S. using it regularly,” said Sabandal, research assistant professor in UTEP’s Department of Biological Sciences. “Given caffeine’s popularity, we wanted to explore whether additional factors influence its impact on behavioral control.”

Experiments Show Night vs. Day Differences in Fly Behavior

To examine the effects of caffeine more closely, the researchers carried out multiple experiments in which fruit flies were given caffeine in their diets under different circumstances. These conditions included varying caffeine concentrations, consumption during the day or at night, and combinations with sleep deprivation. The scientists measured impulsivity by observing whether the flies could stop moving when exposed to strong airflow, which is naturally unpleasant for them.

“Under normal circumstances, flies stop moving when exposed to strong airflow,” said Saldes, now a science research specialist at the University of Illinois College of Medicine Peoria and a former doctoral student at UTEP. “We found that flies consuming caffeine at night were less able to suppress movement, displaying impulsive behaviors such as reckless flying despite these aversive conditions.”

In contrast, the researchers reported that caffeine consumed during the daytime did not trigger the same type of reckless flying.

A team of UTEP biologists have discovered that nighttime caffeine consumption can increase impulsive behavior, potentially leading to reckless actions. The study examined how nighttime caffeine intake affects inhibition and impulsivity in fruit flies. On the left: Fruit flies are exposed to strong airflow without caffeine intake, demonstrating little to no movement, depicting robust inhibitory control. On the right, fruit flies that have ingested caffeine at night demonstrate reckless flying and inadequate movement suppression in response to strong airflow. Credit: The University of Texas at El Paso

Sex Differences and Implications for Night Workers

The experiments also revealed clear differences between male and female flies. Even though both sexes had similar caffeine levels in their bodies, females showed much stronger increases in impulsive behavior after consuming caffeine.

“Flies don’t have human hormones like estrogen, suggesting that other genetic or physiological factors are driving the heightened sensitivity in females,” said Biological Sciences Professor Kyung-An Han. “Uncovering these mechanisms will help us better understand how nighttime physiology and sex-specific factors modulate caffeine’s effects.”

The researchers caution that the findings may be particularly relevant for people who rely on caffeine while working overnight, including shift workers in fields such as health care and the military. The results also suggest females may be more sensitive to the behavioral effects of nighttime caffeine.

Reference: “Nighttime caffeine intake increases motor impulsivity” by Erick Benjamin Saldes, Paul Rafael Sabandal and Kyung-An Han, 24 July 2025, iScience.
DOI: 10.1016/j.isci.2025.113197

Never miss a breakthrough: Join the SciTechDaily newsletter.
Follow us on Google and Google News.