A groundbreaking study from the University of Texas at El Paso (UTEP) has revealed that consuming caffeine at night can significantly heighten impulsive behavior, with women potentially facing a greater risk than men. Conducted by researchers in UTEP's Department of Biological Sciences, the findings shed new light on how the timing of caffeine intake influences decision-making and self-control, offering critical insights for night-shift workers, students pulling all-nighters, and higher education professionals managing irregular schedules.

The research, published in the prestigious journal iScience on July 24, 2025, utilized the fruit fly model (Drosophila melanogaster)—a staple in neuroscience due to its conserved neural pathways with humans—to demonstrate that nighttime caffeine disrupts inhibitory control. This leads to reckless actions, such as failing to avoid aversive stimuli, even when caffeine blood levels match daytime consumption.

UTEP's Innovative Approach to Unraveling Caffeine Mysteries

At the heart of this discovery is UTEP's thriving Biological Sciences department, known for its focus on neurobiological mechanisms underlying behavior, addiction, and gene-environment interactions. Lead investigator Kyung-An Han, Ph.D., a professor whose lab explores dopamine signaling and behavioral plasticity, collaborated with Paul R. Sabandal, Ph.D., research assistant professor, and former doctoral student Erick B. Saldes, now at the University of Illinois College of Medicine Peoria.

Han’s lab has a track record of impactful work, including NIH-funded projects totaling millions for addiction research. Recently, UTEP secured $2.8 million from the National Institutes of Health to study addiction-related decision-making, underscoring the university's commitment to translational neuroscience. This caffeine study exemplifies how UTEP leverages border-region resources and diverse talent to tackle global health challenges, positioning it as a leader in Hispanic-serving institution research.

Decoding the Experiment: How Flies Mimic Human Impulsivity

The study employed a go/no-go paradigm to measure motor impulsivity. Flies, aged 4-5 days, were fed caffeine-laced food (1-10 mg/mL) during nighttime (Zeitgeber time 12-24) or daytime (ZT0-4). Impulsivity was quantified as 'loss of inhibition events' (LIEs)—movements exceeding 60 mm/s under strong airflow, a natural aversive stimulus that normally halts activity.

Results were striking: Nighttime caffeine caused dose-dependent LIE increases, peaking at higher doses. Females showed 2-3 times more impulsivity than males despite equivalent or lower caffeine levels, confirmed via ELISA assays. Daytime feeding produced no effect, isolating the circadian component.

This fruit fly model is highly relevant to humans; dopamine pathways regulating reward, motivation, and inhibition are conserved across species. Drosophila's mushroom body (MB)—analogous to human basal ganglia—processes these signals similarly, making it ideal for dissecting impulsivity linked to disorders like ADHD or addiction.

Sex-Specific Vulnerabilities: Females More Susceptible

A pivotal finding was the pronounced sex difference. Female flies exhibited heightened sensitivity, prompting Han to note, 'Flies don’t have human hormones like estrogen, suggesting other genetic or physiological factors.' This aligns with human data where women report stronger negative caffeine effects like anxiety, while men experience vigor.

In the US, women dominate night-shift roles in healthcare (over 80% of nurses) and education support, where caffeine is ubiquitous—88% of US nurses drink coffee daily. With 20% of the workforce (32 million) on shifts, and women comprising 47% of night workers, these insights urge tailored guidelines.

UTEP's border location amplifies relevance; many students and faculty juggle shifts in El Paso's service economy, highlighting higher ed's role in addressing local health disparities.

Photo by Ichsan Chairul on Unsplash

Dopamine: The Circadian Conductor of Impulsivity

Mechanistically, dopamine D1 receptor (dDA1/Dop1R1) in MB α/β and γ lobes mediated effects, with γ lobes hypersensitive. Silencing protocerebral anterior medial (PAM) dopaminergic neurons or reducing synthesis blocked impulsivity; excess dopamine exacerbated it. D2 receptors were uninvolved, pinpointing D1 specificity.

Not sleep loss—vortexing or light disruption mimicked no effect—nor hyperactivity (unchanged walking speed). Circadian gating via dopamine explains why timing trumps dose, echoing human studies on chronobiology and addiction vulnerability.

Sabandal emphasized, 'Caffeine’s popularity—85% US adults daily—warrants exploring modulators like time-of-day.'

Real-World Ramifications for Higher Ed and Beyond

Shift work disorder affects 10-40% of workers, impairing cognition and safety. Military personnel (high caffeine users, avg 285-400mg/day) and healthcare staff face risks; impulsive errors in high-stakes environments could rise.

For universities, late-night studying or TA shifts mirror this; 64% US adults caffeinate daily, often evenings. UTEP's findings advocate timing awareness, especially for female-dominated fields like nursing programs.

Read the full study in iScience.

UTEP's Rising Star in Neuroscience Research

UTEP's Biological Sciences boasts NIH BUILDing SCHOLARS ($15.2M), cancer research ($6.1M), and CAREER awards. Han's lab exemplifies this, bridging basic science to societal impact amid UTEP's Hispanic-majority student body (80%+), fostering inclusive research.

More at UTEP News.

Caffeine Culture in America: Stats and Habits

94% Americans consume caffeinated drinks; average 135-400mg/day. Shift workers up non-work intake 72-169%. Healthcare/military: 88-90% daily coffee. Women, overrepresented, may amplify risks per UTEP data.

Human studies link high habitual caffeine to impulsivity traits, though acute effects mixed; UTEP pioneers timing focus.

Photo by Jorge Franganillo on Unsplash

Bridging Flies to Humans: Why Models Matter

Fruit flies share 75% disease genes with humans; MB dopamine mirrors striatum. Studies validate fly impulsivity for ADHD, addiction. UTEP advances this, informing clinical trials.

Practical Advice and Future Outlook

Limit nighttime caffeine; opt decaf post-sunset. Shift workers: strategic timing, monitor sex differences. UTEP eyes human validation, chronotherapy.

Explore UTEP opportunities via faculty positions or research roles.

• Track intake with apps.
• Prioritize sleep hygiene.
• Consult for personalized advice.