Summary: A six-week study of financial traders found an unexpected interaction between alcohol and caffeine: when used together, these substances can mask each other’s negative effects on sleep, creating a cycle of perceived recovery that may encourage continued use despite actual harm.
Over six weeks, participants kept daily logs of their caffeine and alcohol intake and reported on their sleep duration and quality. While each substance alone produced the familiar harms—caffeine shortening sleep and alcohol reducing perceived sleep quality—the combined use showed a complex interplay. Daytime caffeine made people feel more alert and masked the perceived effects of alcohol-related sleep disruption, and evening alcohol partially offset caffeine-linked reductions in sleep time. This reciprocal masking can encourage a “self-medication” cycle, where people use one drug to counteract the effects of the other.
Key facts
- Caffeine alone reduced total sleep time by about 10 minutes per cup consumed the previous day.
- Alcohol alone was associated with an average 4% decline in perceived sleep quality per drink.
- When combined, caffeine and alcohol produced a less straightforward effect: alcohol appeared to blunt the sleep-time reduction associated with caffeine, while caffeine’s alerting effects appeared to mask alcohol’s impact on sleep quality, potentially promoting repeated use.
Source: University of Washington
Overview
This is the first known real-world investigation of how alcohol and caffeine interact night-to-night to influence sleep in a naturally occurring setting. The research, led by teams at the University of Washington School of Medicine’s Center for the Study of Health and Risk Behaviors and UC Berkeley’s Center for Human Sleep Science, focused on a small cohort of financial traders—an occupation characterized by high cognitive demand, irregular hours, and frequent use of stimulants and alcohol to manage stress and performance.
Researchers expected combined use to produce additive harm: caffeine and alcohol each impair sleep in different ways, so together they predicted worse sleep quality and shorter sleep duration. Instead, they found a more nuanced interaction. Frank Song, the study’s lead author and a clinical psychology doctoral candidate at UW, explained that the drugs appeared to obscure one another’s effects. “We anticipated additional declines in subjective sleep quality or duration when both were used,” Song said. “But the interaction actually reduced the measurable negative impact on either quality or quantity, which raised important questions about perception versus objective sleep changes.”
Over time, this pattern can encourage a harmful loop. Evening alcohol suppresses REM sleep and degrades the subjective quality of sleep. People then use daytime caffeine to counteract tiredness, which objectively shortens sleep time. However, because caffeine increases daytime alertness, users may not perceive the decline in sleep quality or quantity, and so continue consuming both substances. The researchers describe this as a potential cycle of self-medication: the sedating effect of alcohol and the stimulating effect of caffeine mask each other’s immediate effects, creating a mismatch between how people feel and how they actually sleep.
The study’s sample of 17 traders recorded daily consumption and sleep-related observations. Results were consistent in showing that caffeine reduced sleep duration by roughly 10 minutes per cup and that each alcoholic drink correlated with a 4% drop in perceived sleep quality. The unexpected finding was the interaction: evening alcohol consumption moderated the sleep-time reductions caused by prior caffeine intake (a statistically significant effect in their analyses), while caffeine’s alerting effect appeared to blunt subjective awareness of alcohol’s disruptive impact on REM sleep.
About this sleep research news
Author: Chris Talbott
Source: University of Washington
Contact: Chris Talbott – University of Washington
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Sleep, alcohol, and caffeine in financial traders” by Frank Song et al., published in PLOS ONE.
Abstract
Sleep, alcohol, and caffeine in financial traders
Alcohol and caffeine are among the most widely used psychoactive substances. Laboratory and epidemiological studies have documented how each affects sleep, but little was known about their night-to-night interaction in everyday life, especially within professions that commonly use both. In a six-week micro-longitudinal study of financial traders, researchers examined how alcohol, caffeine, and their combination influenced nightly sleep.
The study found that alcohol significantly reduced subjective sleep quality, while caffeine primarily reduced sleep quantity. Contrary to the initial hypothesis, the combination produced an interaction: evening alcohol use partially mitigated caffeine’s association with reduced sleep duration. These findings suggest that the sedating properties of alcohol and the stimulant effects of caffeine can obscure one another’s influence on sleep quality and quantity. That obscuring effect may help explain patterns of interdependent use—individuals may use alcohol to combat daytime caffeine effects and use caffeine to counteract alcohol-induced sleepiness—creating a cycle that perpetuates both substances’ use despite negative consequences for sleep.
Understanding these interactions in real-world settings clarifies how commonly used substances influence sleep and performance. The results have implications for occupational health, public health, and the economic costs associated with impaired performance due to disrupted sleep.