Numerous studies have investigated the effects of caffeine ingestion on visual attention. EFSA evaluated a significant number of studies and concluded that caffeine increases both selective attention (focusing on the relevant stimulus) and sustained attention (maintaining focused attention over an extended period of time)1. Based on this evaluation, EFSA concluded that a 75mg serving of caffeine had been demonstrated to increase attention1. Higher caffeine intakes do not necessarily result in additional increases in alertness2-4. It is thought that the relationship between the level of arousal and task performance follows an inverted U-curve, i.e. poor performance can occur due to both under- and over-arousal5. A review published in 2012 suggests that caffeine improves performance on both simple and complex attention tasks, concluding that caffeine has clear beneficial effects on attention, and that these effects are even more widespread than previously assumed6.
In a study comparing the effects of caffeine on attention in non-habitual and habitual caffeine consumers, the effects were dose-dependent in non-habitual caffeine consumers, and the best results for visual attention were achieved with 200mg of caffeine7. Another study found that in habitual consumers, the amount required to enhance both vigilance and visual attention was higher, i.e. 400mg8. In the same manner, caffeine enhanced real-world language processing and improved the rate of detecting errors in discourse. As in other studies, low-caffeine consumers had the highest rates of improvements with 200mg of caffeine, while high consumers’ rates peaked with 400mg9.
The expectation of having consumed caffeine can also affect attention and psychomotor speed10. These findings are in agreement with those of a previous imaging study, which reported that caffeine and expectation of caffeine activate the same brain areas, although less so in the latter case11. At this point the underlying psychological mechanisms of these responses are not clear.
Caffeine and reaction time
The positive effects of caffeine on alertness and reaction time have been studied extensively and an overview of these studies can be found in the EFSA evaluation1.
Other experiments have confirmed the positive effect of caffeine on reaction time, while ‘time perception’ (the sense of time passing in an individual) and ‘time production’ (the time it takes to produce something following a stimulus) appear relatively insensitive to caffeine. Thus, it appears that interval timing and reaction time performance are not always necessarily interdependent12.
Caffeine, alertness and safety in daily-life situations
The effects of caffeine on alertness are often clearest in situations where an individual’s alertness level is reduced, such as when they are suffering from the common cold13, the post-lunch dip14, or working at night15.
- During night work, caffeine has also been shown to reduce cognitive failures and accidents by about half in subjects consuming over 220mg caffeine daily15.
- Moreover, caffeine has been shown to reduce cognitive failures in the non-working population16.
The latter two studies point to the potential impact of caffeine consumption on performance and safety.
Caffeine is often consumed shortly after waking up to help increase alertness and fight sleep inertia.
- Sleep inertia is characterized by a decline in motor dexterity and a subjective feeling of tiredness immediately following an abrupt awakening. Impaired alertness may interfere with the ability to perform mental or physical tasks. Sleep inertia can also refer to the tendency of wanting to return to sleep17.
- Research suggests that caffeine has been shown to help overcome sleep inertia, and one study suggests this may explain, in part, the popularity of drinking caffeine-containing beverages after waking up17.
Finally, the efficacy of drinking coffee versus napping to aid night-time highway driving has been compared.
- Research from one study suggests that drinking one strong coffee (125ml containing 200mg caffeine) is as effective as a 30 minute nap to reduce driving impairment without altering subsequent sleep18.
- A 2012 study found that subjective driving quality during a simulated two hour monotonous highway driving test was significantly improved in the first hour after consuming a single cup of caffeinated coffee containing 80mg of caffeine19.
- Likewise, another study reported that a 30 minute break including a short nap (of less than 15 minutes), or a coffee containing 150-200mg caffeine, were very effective at reducing driver sleepiness. This effect was even more prominent when coffee and a nap were combined20. This amount of caffeine also resulted in reduced driving incidents in a simulated driving test held in the early morning for 30 minutes following no sleep, or about two hours after sleep restriction21.
- A case-control study showed that caffeinated beverages, such as coffee, were associated with a reduced risk of crashing for long-distance commercial motor vehicle drivers22.
- In addition, slow-release caffeine (300mg) has also been reported to reduce lane drifting, speed deviation and accident liability in a simulated driving test23.
- A 2015 study found that drinking caffeinated coffee (providing 150mg caffeine) can reduce levels of drowsiness in drivers by 25%
References for chart:
- Line 1 – No 18 Philip P. et al. (2006) The effects of coffee and napping on nighttime highway driving: a randomized trial. Ann Intern Med, 144:785-91.
- Line 2 – No 19 Mets M.A. et al. (2012) Effects of coffee on driving performance during prolonged simulated highway driving. Psychopharmacology, 222(2):337-42.
- Line 3 – No 20 Horne J. et al. (1999) Vehicle accidents related to sleep: a review. Occup Environ Med, 56:289-94.
- Line 4 – No 23 De Valck E. et al. (2001) Slow-release caffeine as a countermeasure to driver sleepiness induced by partial sleep deprivation. J Sleep Res, 10:203-9.
- Line 5 – No 24 Bosó P. et al. (2015) Coffee in the driving of vehicles: analysis of its effects on tired or drowsy drivers. http://alvolantegentedespierta.es/#homePage
Data from the aforementioned studies suggests that caffeine may help as an effective countermeasure to the performance decrements induced by sleep-deprivation, particularly when there is no opportunity to take a nap.
However, it must be noted that, while caffeine intake at a level of 200-400mg may increase alertness and reduce reaction time after alcohol ingestion, alcohol-induced impairment in drivers will not be counteracted by caffeine25. Drivers should always follow existing road safety guidelines.