If you often rely on coffee to stay awake during night shifts—or simply enjoy an evening cup—it’s time to stop. Unfortunately, this habit can have unexpected negative effects on your body.
For many people, a morning cup of coffee is the perfect way to start the day, while others see an afternoon brew as their secret to staying alert. However, drinking coffee at the wrong time can reduce its positive effects—or even harm your health.
The Risks of Late-Night Coffee
A study by researchers at the University of Texas at El Paso (USA), recently published in iScience, examined how caffeine affects the brain at night and whether its impact differs depending on the time of day.
Speaking to Newsweek, co-author Kyung-An Han explained: “We know caffeine affects people differently. In this study, we demonstrated that even for the same person, caffeine’s effects are not uniform—timing is crucial.”
The researchers used fruit flies—a species with genetic and neurological traits similar to humans—to explore the relationship between coffee and behavior. Caffeine was added to the flies’ diet at different doses: some consumed it at night and others during the day, combined with sleep deprivation.
They then tested how the flies responded to a strong air current, focusing on their ability to stop moving—a typical reflex when exposed to wind or gusts. The results revealed that flies given caffeine at night “were less able to suppress movement and even showed impulsive behavior, such as flying recklessly despite the uncomfortable conditions.”
Interestingly, this effect was not observed in flies that consumed caffeine during the day.
According to Han, these findings challenge the common belief that caffeine is simply a stimulant. Instead, the researchers define it as a behavioral modulator with specific time- and sex-related vulnerabilities. “Our next important step is to analyze the molecular and neural mechanisms behind caffeine’s time- and sex-dependent vulnerabilities,” Han said.
Moreover, in their study, female flies displayed significantly higher nighttime caffeine-induced impulsivity than males. Since fruit flies do not have human estrogen, this suggests that other genetic or physiological factors drive this heightened sensitivity.
The research team noted that these findings should serve as a warning for night-shift workers—such as healthcare professionals and military personnel—to reconsider their late-night habits of drinking coffee or energy drinks.