It’s the late shift. You’re tired. You pour another cup of coffee. But what if that midnight jolt of caffeine isn’t just keeping you awake — it’s making you reckless?
New research from The University of Texas at El Paso (UTEP) has uncovered that caffeine consumed at night impairs the brain’s ability to regulate impulsive behavior — at least in fruit flies. The findings raise new questions about caffeine’s effects on human behavior when consumed at odds with our biological clocks.
“We found that flies consuming caffeine at night were less able to suppress movement,” said lead author Erick Saldes. “They displayed impulsive behaviors such as reckless flying despite these aversive conditions.”
Why Fruit Flies?
The study used Drosophila melanogaster, a fruit fly species often leveraged in behavioral neuroscience. While they might seem like an unlikely stand-in for humans, these tiny insects share many genes and neural pathways with us, especially in areas controlling behavior, sleep, and neurotransmission.
That’s why Kyung-An Han, a professor at UTEP and senior author of the paper, sees them as ideal subjects: “Flies don’t have human hormones like estrogen, suggesting that other genetic or physiological factors are driving the heightened sensitivity in females.”
And yes — female flies reacted more impulsively than males to nighttime caffeine, despite similar internal caffeine levels.
A Daily Ritual with a Darker Nighttime Side
Researchers introduced caffeine into the flies’ diets at different times of the day. They then tested their ability to suppress movement in response to strong airflow, a stimulus that normally causes flies to freeze.
During the day, caffeine had no effect. But at night, the caffeinated flies failed the test.
Their behavior was erratic. They flew recklessly instead of staying still, a response researchers interpret as a breakdown in inhibitory control — a core function of executive brain processes.
Importantly, caffeine-induced impulsivity wasn’t simply a byproduct of restlessness. The flies didn’t walk faster or more often. Nor was it due to sleep deprivation alone. When researchers deprived flies of sleep through other methods — like flashing light or mechanical vibration — the same impulsive behavior did not occur.
Timing Matters More Than You Think
Caffeine didn’t impair behavior when consumed during the day, even when internal caffeine levels were matched to nighttime levels. This suggests that the timing of caffeine intake — not the dose — determines its effect on impulse control.
“Nighttime caffeine intake causes impulsivity in flies, with females more affected,” the researchers wrote. “This effect is circadian-dependent, not due to hyperactivity or sleep loss.”
This points to something scientists have long suspected: caffeine interacts with the brain’s internal clock, or circadian rhythm. Human studies have shown similar trends. For example, late-evening caffeine can disrupt sleep architecture and delay melatonin release. In contrast, caffeine consumed in the morning improves attention and motor function.
So, what’s happening in the brain when caffeine triggers impulsivity?
It turns out the behavior is deeply tied to dopamine, the brain’s chemical messenger involved in reward, arousal, and control.
The researchers zeroed in on a brain structure in flies called the mushroom body. This brain structure is analogous to the human prefrontal cortex, which governs executive function. They found that dopamine signaling through the D1 receptor (dDA1) in specific regions of the mushroom body (the α/β and γ lobes) was essential for caffeine-induced impulsivity.
In flies with a mutation that impairs dopamine production, the impulsive behavior disappeared — even after caffeine. The same was true when scientists silenced dopamine-producing neurons or blocked the D1 receptor.
When they overexpressed the D1 receptor in certain brain regions, the impulsivity actually intensified, especially in the γ lobes.
“This clearly shows that dopamine — and specifically the D1 receptor — is both necessary and sufficient for this effect,” said Saldes.
Why This Matters for Humans
If fruit flies are any guide, our biology may be wired to tolerate caffeine differently depending on the time of day — and our sex.
The implications are especially important for people who work overnight shifts: healthcare workers, truck drivers, military personnel, and others who often depend on caffeine to stay alert.
“Given caffeine’s popularity, we wanted to explore whether additional factors influence its impact on behavioral control,” said Paul Sabandal, co-author of the study. “These findings could have negative implications for shift workers, particularly females.”
In humans, women are more likely to report anxiety and negative mood effects after consuming caffeine. Men, on the other hand, often experience increased energy and focus. This study helps build a foundation for understanding the neurobiology behind those differences.
And while flies don’t have human hormones like estrogen, the sex-specific response hints at deeper physiological variables — perhaps genetic or metabolic — that influence how caffeine acts on the brain.
There are caveats though. The flies consumed caffeine in groups, meaning individual intake likely varied. The exact time during the night when caffeine causes the greatest effect remains unknown. And the model doesn’t replicate all the complexities of human behavior and metabolism.
Still, the findings are compelling. They raise practical questions about how we use caffeine — not just how much, but when.
So Should You Skip That Late-Night Coffee?
Not necessarily. But this research adds to a growing body of evidence that caffeine’s effects are not universal across the day.
If you find yourself acting on impulse or making poor decisions during a night shift, your coffee may not be helping in the way you think.
More research in humans is needed. But for now, the takeaway is simple: Caffeine doesn’t just wake you up. Sometimes, it wakes up the wrong parts of your brain.
The findings appeared in the journal iScience.
