For many people, coffee is more than just a drink. It’s a source of comfort, a daily habit, and a quick energy boost to get through late nights or long shifts. But new research suggests that drinking coffee at night could do more than just keeping you awake. A study by the University of Texas at El Paso (UTEP) found that nighttime caffeine intake can increase the risk of reckless behaviour, especially in women.
The study, published in the journal iScience, used fruit flies (Drosophila melanogaster) to explore the link between caffeine and impulsive behaviour. While fruit flies may seem like an unusual choice, scientists often use them to study human-like behaviours because of their genetic and neural similarities to us.
Study findings
Researchers introduced caffeine into the flies’ diets under different conditions, testing varying doses, nighttime versus daytime consumption, and even the effects of caffeine combined with sleep deprivation.
To measure impulsivity, they exposed the flies to strong airflow, a naturally unpleasant stimulus that typically causes them to stop moving. But the results showed something surprising.
“Under normal circumstances, flies stop moving when exposed to strong airflow,” explained Erick Saldes, a science research specialist at the University of Illinois College of Medicine Peoria. “We found that flies consuming caffeine at night were less able to suppress movement, displaying impulsive behaviours such as reckless flying despite these aversive conditions.”
Further, flies that consumed caffeine during the day did not show the same reckless behaviour.
Why women may be more vulnerable
Another key finding: females were more prone to caffeine-induced impulsivity than males, even though both had similar caffeine levels in their bodies.
“Flies don’t have human hormones like oestrogen, suggesting that other genetic or physiological factors are driving the heightened sensitivity in females,” said Kyung-An Han, Professor of Biological Sciences. “Uncovering these mechanisms will help us better understand how nighttime physiology and sex-specific factors modulate caffeine’s effects.”
Why it matters
The findings raise concerns for people who regularly consume caffeine at night, particularly shift workers, healthcare staff, and military personnel, where staying alert is critical, but so is good decision-making.
While more research in humans is needed, the study adds to growing evidence that timing matters when it comes to caffeine. That late-night coffee might not just affect your sleep, it could influence how you act.
