Caffeine and mental alertness – part 1

Print this page

Caffeine and visual attention

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 (focussing on the relevant stimulus) and sustained attention (maintaining focused attention over an extended period of time)1. A 75mg serving of caffeine, the amount found in around one regular cup of coffee, has been demonstrated to increase attention1.  Higher caffeine intakes, such as those found in more than one or two cups of coffee, do not necessarily result in additional increases in alertness2,3. It is thought that the relationship between the level of arousal and task performance follows an inverted U-curve i.e. performance decrements can occur due to both under- and over-arousal4. 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 assumed5.

In 2010, in a series of experiments by the same research group, the effects of caffeine on attention were compared in non-habitual and habitual caffeine consumers. In non-habitual caffeine consumers, the effects were dose-dependent and the best results for visual attention were achieved with 200mg of caffeine (the equivalent of around two cups of coffee)6. In habitual consumers, the amount required to enhance both vigilance and visual attention was higher i.e. 400mg7(around four cups of coffee). In the same manner, caffeine enhanced real-world language processing and improved the rate of detecting errors in discourse. As in the previous studies low-caffeine consumers had the highest rates of improvements with 200mg of caffeine while high consumers’ rates peaked with 400mg8.

The expectation of having consumed caffeine can also affect attention and psychomotor speed9. These findings are in agreement with those of an older imaging study, reporting that caffeine and expectation of caffeine activate the same brain areas, but in a more limited way in the latter case10. At this point the underlying psychological mechanisms of these responses are not clear.

Caffeine and reaction time

The positive effects of caffeine on reaction time have been studied extensively over the last decades. An overview of the studies can be found in the EFSA evaluation1.

Other experiments have confirmed the positive effect of caffeine on reaction time, while ‘time perception’* and ‘time production’** appear relatively insensitive to caffeine. Thus, it appears that interval timing and reaction time performance are not always necessarily interdependent11.

* the sense of time passing in an individual
**the time it takes to produce something following a stimulus

Caffeine, alertness and safety in daily-life situations

The effects of caffeine on alertness are most often clearest in situations where an individual’s alertness level is reduced, such as when they are suffering from the common cold12, the post-lunch dip13 or working at night.

  • During night work, caffeine has also been shown to reduce cognitive failures and accidents by about half in subjects consuming over 220mg caffeine daily, i.e. the amount found in approximately two cups of coffee14.
  • Moreover, caffeine reduces cognitive failures in the non-working population15.
  • The latter two studies point to the benefits of caffeine consumption on performance and safety.

Caffeine is often consumed at awakening to increase alertness and fight sleep inertia.

  • Sleep inertia is characterised by a decline in motor dexterity and a subjective feeling of grogginess immediately following an abrupt awakening. The impaired alertness may interfere with the ability to perform mental or physical tasks. Sleep inertia can also refer to the tendency of a person wanting to return to sleep.
  • Caffeine has been shown to overcome sleep inertia, which may explain, in part, the popularity of caffeine-containing beverages after waking up16.

Finally, the efficacy of coffee versus napping on night time highway driving has been compared.

  • It appears that drinking one strong coffee (125ml containing 200mg caffeine) is as effective as a 30 minute nap to reduce driving impairment without altering subsequent sleep17.
  • 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 (80mg of caffeine)18.
  • Likewise, another study reported that a 30 minute break including a short nap (less than 15 minutes), or a coffee containing 150-200mg caffeine, were very effective. This effect was even more prominent when the two were combined19. This amount of caffeine also resulted in reduced driving incidents in a simulated driving test, in the early morning for 30 minutes following no sleep, or about two hours after sleep restriction20.
  • A case-control study showed caffeinated beverages, such as coffee, to be associated with a reduced risk of crashing for long distance, commercial motor vehicle drivers21.
  • In addition, slow-release caffeine (300mg) has also been reported to reduce lane drifting, speed deviation and accident liability in a simulated driving test22.

These data suggest that caffeine can serve 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 (200-400mg, i.e. the equivalent of 2-4 cups of coffee) may increase alertness and reduce reaction time after alcohol ingestion, alcohol-induced impairment will not be counteracted by caffeine in drivers23.

Further information on coffee consumption and driving is available here.

This information is intended for Healthcare professional audiences.
Please consider the environment before printing.