For many, coffee is more than just a beverage — it’s a comforting ritual, a source of energy, and a companion during late nights or long work shifts. However, new research suggests that drinking coffee at night could do more than just keep you awake; it might influence your behaviour. A study conducted by the University of Texas at El Paso (UTEP) and published in the journal iScience reveals that nighttime caffeine consumption may increase the risk of impulsive behaviour, particularly among women.

The researchers used fruit flies (Drosophila melanogaster) — commonly used in behavioural studies due to their genetic and neurological similarities to humans — to examine how caffeine affects decision-making. They introduced caffeine into the flies’ diets under different scenarios, such as varying doses, times of consumption (day vs. night), and combinations with sleep deprivation.

To test impulsivity, the flies were exposed to a strong airflow — a stimulus that normally causes them to freeze or stop movement. Surprisingly, flies that consumed caffeine at night displayed reckless flying behaviour, ignoring the aversive stimulus. “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 despite the conditions.”

Interestingly, flies that had caffeine during the day did not show the same reaction. Furthermore, female flies exhibited stronger impulsive responses than males, even though both had similar levels of caffeine in their systems. According to Kyung-An Han, Professor of Biological Sciences, this suggests that other genetic or physiological factors, rather than hormones like oestrogen, may be contributing to this heightened sensitivity in females.

These findings could have real-world implications for individuals who frequently consume caffeine at night, such as shift workers, healthcare professionals, and military personnel. While more human-based studies are needed, the evidence indicates that the timing of caffeine intake matters. Late-night coffee might not only disrupt sleep but could also impair judgement and self-control, especially in women.