06-08-2025
What Happens When You Drink Coffee at Night
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Bad news for people who work late shifts—as drinking coffee at night may lead to impulsive behavior, a new study has warned.
Plenty of people will declare they can't start their morning without a cup of coffee to wake them up—but what happens if you drink it when your brain should be unwinding in preparation for sleep?
A study from researchers at the University of Texas at El Paso, now published in iScience, aimed to discover exactly what caffeine does to your brain at night, and whether there was any noticeable difference to drinking it at different times of the day.
Paper author and biologist Paul Sabandal said in a statement that due to caffeine's popularity, he and his colleagues "wanted to explore whether additional factors influence its impact on behavioral control."
It is estimated that as many as 85 percent of Americans may drink coffee daily—with more than two billion cups thought to be drunk each day around the world.
Stock image of a man drinking coffee at night while working.
Stock image of a man drinking coffee at night while working.
Tirachard/Getty Images
Researchers used a fruit fly species—which has genetic and neural parallels with humans—to study the relationship between coffee and behavior, conducting a series of experiments introducing caffeine to the flies' diets.
These included different caffeine doses, some at night and some during the day, and in combination with sleep deprivation.
Researchers then tested the flies' reactions in response to strong airflow, in particular their ability to stop movement, which the insects would usually do when exposed to wind or strong airflow.
Lead researcher Erick Saldes revealed in a statement that the flies who consumed caffeine at night were "less able to suppress movement, displaying impulsive behaviors such as reckless flying despite these aversive conditions."
Interestingly, this effect was not seen in the flies that ingested caffeine during the day.
Female flies also showed significantly greater nighttime caffeine-induced impulsivity than males—but as flies do not have the human hormone estrogen, it suggested other genetic or physiological factors were driving this heightened sensitivity.
The team has warned that the study's findings could have negative implications for those who work late shifts, such as health care workers and military personnel, who are more likely to consume caffeine at night.
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Reference
Saldes, E. B., Sabandal, P. R., & Han, K.-A. (2025). Nighttime caffeine intake increases motor impulsivity. iScience, 28(8).
