UTEP study: Drinking coffee at night increases impulsive behavior

EL PASO, Texas (Aug. 5, 2025) – Do you find yourself reaching for a cup of coffee at night to stay alert? A new study from The University of Texas at El Paso suggests you might want to reconsider. A team of UTEP biologists have discovered that nighttime caffeine consumption can increase impulsive behavior, potentially leading to reckless actions.

The study, published in iScience, examined how nighttime caffeine intake affects inhibition and impulsivity in fruit flies and was led by Erick Saldes, Ph.D., Paul Sabandal, Ph.D., and Kyung-An Han, Ph.D. Drosophila melanogaster, the fruit fly species used in the study, is a powerful model to study complex behaviors due to its genetic and neural parallels with humans, said Han.

“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.”

The team designed a series of experiments introducing caffeine into the flies’ diets under various conditions, including different caffeine doses, nighttime versus daytime consumption and in combination with sleep deprivation. The team then assessed impulsivity by measuring the flies’ ability to suppress movement in response to strong airflow, a naturally unpleasant stimulus.

“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.”

Interestingly, caffeine consumed by the flies during the daytime did not lead to the same reckless flying, the team said.

The team also discovered notable sex differences. Despite having comparable levels of caffeine in the body, females exhibited significantly greater caffeine-induced impulsivity than males.

“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 team warns that the findings could have negative implications for shift workers, health care and military personnel who consume coffee at night, particularly females.

This study was conducted in Han’s lab within UTEP’s Department of Biological Sciences. The lab focuses on the neurobiological basis of behavioral plasticity including learning, memory and addiction as well as gene-by-environment interactions relevant to Alzheimer’s disease and related dementia.

About The University of Texas at El Paso

The University of Texas at El Paso is America’s leading Hispanic-serving university. Located at the westernmost tip of Texas, where three states and two countries converge along the Rio Grande, 84% of our 25,000 students are Hispanic, and more than half are the first in their families to go to college. UTEP offers 171 bachelor’s, master’s and doctoral degree programs at the only open-access, top-tier research university in America.