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Coffee at night may increase risky behavior, especially in women

That late-night coffee might make you more impulsive—and the effect could hit women harder.

Date:

April 1, 2026

Source:

University of Texas at El Paso

Summary:

Drinking coffee at night might come with an unexpected downside: increased impulsivity. In a new study, fruit flies given caffeine after dark behaved more recklessly, ignoring signals that normally make them stop. Daytime caffeine didn’t have the same effect, pointing to a strong link between timing and behavior. Even more striking, females were far more sensitive than males.

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Nighttime caffeine may push the brain toward impulsive, riskier behavior, according to new research. The effect was especially strong in females and didn’t appear when caffeine was consumed during the day. Credit: Shutterstock

Do you reach for coffee at night to stay awake? New research from The University of Texas at El Paso suggests that habit may come with an unexpected downside. Scientists found that consuming caffeine at night can increase impulsive behavior, raising the likelihood of risky or uncontrolled actions.

The findings, published in iScience, come from a study led by Erick Saldes, Ph.D., Paul Sabandal, Ph.D., and Kyung-An Han, Ph.D. The researchers investigated how caffeine consumed at different times of day influences inhibition and impulsivity using fruit flies.

The species used, Drosophila melanogaster, is widely used in behavioral research because its genetic makeup and neural systems share important similarities with humans, Han explained.

"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."

Nighttime caffeine linked to reduced self control

To test this, the team fed fruit flies caffeine under different conditions, including varying doses, daytime versus nighttime exposure, and combinations with sleep deprivation. They then evaluated impulsivity by observing how well the flies could stop moving when exposed to strong airflow, which they naturally find unpleasant.

"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."

Flies that consumed caffeine during the day did not show the same pattern, indicating that the timing of caffeine intake plays a key role.

Female flies show stronger response to caffeine

The researchers also observed clear differences between males and females. Even though both sexes had similar levels of caffeine in their systems, females displayed much higher levels of impulsive behavior.

"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."

Potential implications for night shift workers

The findings may have broader implications for people who rely on caffeine at night, including shift workers, health care professionals, and military personnel. The researchers note that these effects could be especially relevant for women.

The study was carried out in Han's lab in 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 linked to Alzheimer's disease and related dementia.