Good news coffee drinkers: caffeine reduces the effect of drowsiness on cognitive performance

The molecular and physiological action of caffeine in the human nervous system has been well-established. The main mechanism includes an increase in cyclic adenosine monophosphate (cAMP) — a crucial intracellular signalling molecule. In the noradrenergic nervous system, cAMP magnifies noradrenaline’s stimulatory action on the cardiovascular system and the brain. While the former is required to increase the blood supply to skeletal muscle, thereby enhancing physical performance, the latter introduces improved alertness, arousal, and memory. Research shows that caffeine also decreases subjective fatigue, mental exhaustion, and irritability. Drowsiness — a separate state between wakefulness and sleep — is a common consequence of sleep deprivation. Maintaining cognitive performance, such as fast reaction time and error avoidance, is often challenging for drowsy individuals. While not being particularly dangerous when reading a book or watching TV, impaired cognitive performance is obviously an issue when driving a car. The University of Sydney and University of South Australia collaborated to investigate caffeine’s effects on drowsiness and cognitive performance in the recent February issue of Nature Scientific Reports. 

50-hour sleep deprivation study

The research team analyzed the effect of caffeine  in a double-blind, placebo-controlled study: participants were allowed a generous 10-hour sleep, but had to stay awake for 50 hours right after to introduce sleep deprivation conditions. Individuals were administered 200 mg of caffeine or a placebo chewing gum at specific intervals at the subjective night-time. During the 50-hour sleep deprivation, participants were monitored for drowsiness levels and performance in cognitive tasks with varying complexity.

As expected, the researchers observed that caffeine reduced  sleepiness compared to the placebo group, however, this reduction was modest: the caffeinated group reached an average score of 5.4 of 10 in the Johns Drowsiness Scale — anything above 5 is considered a high-risk for performance failure. There was thus only a slight improvement compared to the placebo group, which averaged at 7.9. Caffeine was thus unsuccessful at preventing marked drowsiness. Further, chronological patterns of reaction times in all cognitive tests were similar in both groups — even after 10 hours of continuous wakefulness.

So, did caffeine help?  The researchers found that the linear relationship between drowsiness and cognitive performance was disrupted by caffeine. The stimulant drug effectively de-coupled cognitive performance from sleepiness: reaction times  were enhanced, while errors and false alarms were significantly reduced. Caffeine improved cognitive performance: individuals supplied with caffeine performed given tasks more quickly and precisely than the placebo group, even though their drowsiness levels were similar. The disruption qualifies caffeine as an effective countermeasure for drowsiness degradative effects. 

The more complex task, the stronger the effect

The Australian research team also found that the effect of caffeine was amplified by increased task complexity: reaction times were shorter and there were fewer errors by caffeine-treated subjects  in complex cognitive tasks than in relatively simple tasks. The same research team previously showed performance on simpler tasks is highly sensitive to sleep loss, hence caffeine’s capacity to enhance vigilance when sleep deprived is quite limited. The good news is that response inhibition and decision making appear to be less dependent on total sleep loss and can be enhanced by caffeine. Such findings also suggest declined performance on complex cognitive tasks is not solely dictated by levels of drowsiness.

A hope for road accident reduction?

Research on the effect of caffeine on cognitive performance and alertness could be inferred onto everyday tasks such as driving. Drowsy driving contributes to around  20% of all car accidents in the United States and Europe. Further, JAMA Pediatrics published that sleep deprived young adults were around 20 percent more likely to be involved in car accidents to non-sleep-deprived peers; while accounting for confounders such as number of driving hours per week or history of car crashes.

Driving undoubtedly demands high cognitive performance. The finding that cognitive performance is de-coupled from drowsiness is promising for the prevention of road accidents. Caffeine chewing gum, as utilised in the experiment, is quickly absorbed, and is safely and easily administrable. The effectiveness of coffee in carrying out these caffeine-induced benefits is yet to be determined. Presumably, caffeine administration at strategically timed intervals could reduce the impact of drowsiness on cognitive performance in everyday life, alleviating the burden of drowsiness-induced road accidents.


Aidman, E., Balin, M., Johnson, K. et al. (2021) ‘Caffeine may disrupt the impact of real-time drowsiness on cognitive performance: a double-blind, placebo-controlled small-sample study’, Scientific Reports, 11(1):4027. doi: 10.1038/s41598-021-83504-6.

Aidman, E., Jackson, S. A. & Kleitman, S. (2019) ‘Effects of sleep deprivation on executive functioning, cognitive abilities, metacognitive confidence, and decision making’, Applied Cognitive Psychology, 33(2), pp. 188–200. doi: 10.1002/acp.3463.

Martiniuk, A. L. C., Senserrick, T., Lo, S. et al. (2013) ‘Sleep-Deprived Young Drivers and the Risk for Crash: The DRIVE Prospective Cohort Study’, JAMA Pediatrics, 167(7), pp. 647–655. doi:10.1001/jamapediatrics.2013.1429

European Union (2021) Frequency of fatigue-related crashes. Available at: (Accessed: 1 May 2021).

Alexandra Lesayova is a 2nd year Biomedical Sciences student. Find her on LinkedIn @Alexandra Lesayova or Twitter @ALesayova

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