There are a large number of studies showing that caffeine increases

alertness. Effects of caffeine on alertness are often more easy to

demonstrate in subjects in a state of low alertness. Thus caffeine

can counteract the effects on alertness of benzodiazepine

administration, the early morning, night-time working, a cold or

sleep loss. However, caffeine has also been shown to increase

alertness in fully alert subjects. Effects of caffeine on alertness have

been demonstrated both at high doses as well as at more realistic

lower doses. It has also been shown that the effects are due to

caffeine itself rather than the combination of caffeine and the drink in

which it is presented. These effects on alertness have important

implications for certain practical situations as safety is often at risk

when alertness is low. Improved alertness can be beneficial at

different times of the day and night, and whilst undertaking different tasks. Driving is now somewhat essential for may people, and driver fatigue is a significant cause of road traffic accidents. In 2006 researchers investigated the effects of coffee and napping in relation to driver alertness, conducted on real nightime driver performance. The participants consumed 125ml coffee containing 200mg caffeine before driving 200 killometres between 6.00 and 7.20 pm, or 2.00 and 3.30 am. The authors concluded that coffee or napping at night statistically significantly reduces driving impairment without altering subsequent sleep.

In a 1992 review of the literature, it was concluded that “it appears that caffeine can increase anxiety when administered in single bolus doses of 300 mg or higher, which is many times greater than the amount present in a single serving of a typical caffeine-containing beverage. However, in lower doses it appears to have little effect on this mood-state or, under certain circumstances, it may even reduce anxiety levels.” 

Since this review was published two studies have confirmed that high doses of caffeine promote anxiety. Thus 600 mg caffeine or several cups of coffee each containing 125 mg caffeine increased anxiety. Caffeinism has been defined as a group of symptoms associated with daily intakes of caffeine of 1000-1500 mg which closely resemble the symptoms of severe chronic anxiety. However, there is little evidence in the literature of associations between caffeine intake and anxiety. Intervention trials with normal doses of caffeine have not shown effects on anxiety either in healthy subjects or in psychiatric patients with anxiety disorders. In addition, caffeine has not been shown to increase the anxiety induced by other stressors. Hence there is no reason to modify the conclusions of the 1992 review on effects of caffeine on anxiety.

In the same 1992 review it was concluded that “When caffeine is consumed in the range of doses found in many foods, it improves the ability of individuals to perform tasks requiring sustained attention, including automobile driving. In addition, when administered in the same dose range, caffeine increases self-reported alertness and decreases sleepiness”. “Adverse behavioural effects occur when caffeine is consumed in excessive doses or by individuals who are overly sensitive to the substance”.

More recent work has confirmed these conclusions. Thus caffeine increases the processing of new stimuli, enhances target detection and response preparation and increases the amount of information processed. Such improvements in mental performance are likely to have important implications for the performance of every day tasks. Thus caffeine improved both performance on a driving simulator and during laboratory tests of attention. Mental performance declines with age. A cross-sectional study of 890 women and 638 men, mean age 73 years, looked for associations between cognitive function as measured by 12 standardised tests and lifetime or current coffee consumption. A higher lifetime coffee consumption in women was significantly associated with better mental performance in six tests.