For many people, a cup of coffee at night feels like a lifesaver, whether it’s to finish a project, power through a late shift, or stay awake on the road.
But new research suggests that this habit may come with hidden risks.
A study from The University of Texas at El Paso (UTEP) shows that drinking coffee at night can make people more impulsive, and the effect appears to be stronger in females.
The study, published in the journal iScience, was led by biologists Erick Saldes, Paul Sabandal, and Professor Kyung-An Han.
To uncover caffeine’s effects, the team turned to an unlikely test subject: fruit flies. While it may sound surprising, fruit flies are often used in neuroscience because their brains share many genetic and functional similarities with humans.
“Caffeine is the most widely consumed psychoactive substance in the world, with about 85% of U.S. adults using it regularly,” explained Sabandal.
“We wanted to explore whether the time of day, sex differences, and other factors influence its impact on behavior and self-control.”
In their experiments, the researchers added caffeine to the flies’ diets under different conditions, including varying doses, day versus night consumption, and in combination with sleep deprivation.
They then measured impulsivity by testing how well the flies could suppress movement when exposed to strong bursts of air, something flies naturally dislike.
Normally, fruit flies stop moving when hit with strong airflow. But the researchers found that flies who consumed caffeine at night behaved differently.
They were less able to stop themselves, showing impulsive and reckless behaviors such as flying directly into the unpleasant wind. By contrast, flies given caffeine during the day did not show this loss of self-control.
One of the most intriguing findings came when the researchers compared male and female flies. Despite having similar levels of caffeine in their systems, female flies were much more affected, displaying greater impulsivity than males.
“Flies don’t have human hormones like estrogen, so this heightened sensitivity in females must come from other genetic or physiological factors,” said Han. “Understanding those mechanisms could give us valuable insight into how nighttime physiology and sex differences shape caffeine’s effects.”
While the study was conducted in fruit flies, the researchers believe the results have important implications for humans.
Many shift workers, health care professionals, and military personnel rely on late-night coffee to stay alert. But if nighttime caffeine increases impulsivity, it could raise the risk of mistakes, accidents, or poor decision-making—especially among women.
The work is part of Han’s lab at UTEP, which studies how genes and environments interact to shape behavior, including learning, memory, addiction, and diseases such as Alzheimer’s.
This latest research adds another layer to the story of caffeine: it may not just be about wakefulness, but also about how self-control shifts depending on when—and who—is drinking it.
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