Elsevier

Food and Chemical Toxicology

Volume 40, Issue 9, September 2002, Pages 1243-1255
Food and Chemical Toxicology

Effects of caffeine on human behavior

https://doi.org/10.1016/S0278-6915(02)00096-0 Get rights and content

Abstract

The literature suggests that the following effects on behavior of adult humans may occur when individuals consume moderate amounts of caffeine. (1) Caffeine increases alertness and reduces fatigue. This may be especially important in low arousal situations (e.g. working at night). (2) Caffeine improves performance on vigilance tasks and simple tasks that require sustained response. Again, these effects are often clearest when alertness is reduced, although there is evidence that benefits may still occur when the person is unimpaired. (3) Effects on more complex tasks are difficult to assess and probably involve interactions between the caffeine and other variables which increase alertness (e.g. personality and time of day). (4) In contrast to the effects of caffeine consumption, withdrawal of caffeine has few effects on performance. There is often an increase in negative mood following withdrawal of caffeine, but such effects may largely reflect the expectancies of the volunteers and the failure to conduct “blind” studies. (5) Regular caffeine usage appears to be beneficial, with higher users having better mental functioning. (6) Most people are very good at controlling their caffeine consumption to maximise the above positive effects. For example, the pattern of consumption over the day shows that caffeine is often consumed to increase alertness. Indeed, many people do not consume much caffeine later in the day since it is important not to be alert when one goes to sleep. In contrast to effects found from normal caffeine intake, there are reports that have demonstrated negative effects when very large amounts are given or sensitive groups (e.g. patients with anxiety disorders) were studied. In this context caffeine has been shown to increase anxiety and impair sleep. There is also some evidence that fine motor control may be impaired as a function of the increase in anxiety. Overall, the global picture that emerges depends on whether one focuses on effects that are likely to be present when caffeine is consumed in moderation by the majority of the population or on the effects found in extreme conditions. The evidence clearly shows that levels of caffeine consumed by most people have largely positive effects on behavior. Excessive consumption can lead to problems, especially in sensitive individuals.

Introduction

The aim of the present article is to review the effects of caffeine on human behavior. The main areas of behavior reviewed are mood, mental performance and sleep. Certain areas, although related to behavior, such as the underlying CNS mechanisms, are not reviewed in detail. This is because most of the research in this area has involved animal studies. These have provided plausible mechanisms for many of the effects observed in humans, but whether specific details (e.g. effects of dose) generalize across species is unknown. Other areas, such as the claims that caffeine is addictive, are not covered here but are discussed in a paper by Dews et al. in this issue.

In all areas it is important to make a distinction between the effects of amounts of caffeine that are normally consumed from food and drinks, and the very different effects observed with excessive amounts or in very sensitive individuals. Unlike other areas of research (e.g. studies of health effects), most studies of the behavioral effects of caffeine have examined acute changes following a single dose. Less is known about effects of regular consumption, although there are now enough data on this topic to draw tentative conclusions. In addition to studying the effects of caffeine consumption, the research has also considered possible changes in behavior as a function of caffeine withdrawal. A critical appraisal of claims that caffeine withdrawal influences performance and mood is provided.

It is important to note that in research reviewed in all subsequent sections, there is considerable variation in the methodology and measuring devices. This has the disadvantage that it is often difficult to compare different studies. However, persistence of effects across a range of methodologies enhances the validity of the reported observations.

Section snippets

Increases in alertness

A large number of studies have shown that consumption of caffeine leads to increased alertness (or reduced fatigue). These effects have often been demonstrated using paradigms involving low alertness situations (e.g. following administration of benzodiazepines—Johnson et al., 1990; early morning—Smith et al., 1992; working at night—Smith et al., 1993a; when the person has a cold—Smith et al., 1997a; and sleep loss—Bonnet et al., 1995). However, beneficial effects of caffeine have been

Studies before 1990

There are already a number of extensive reviews (e.g. Lieberman, 1992) which have summarized early studies of the effects of caffeine on human performance. This section outlines the main points to emerge from such reviews, and is then followed by a critical evaluation of more recent research in this area.

Effects of caffeine on sleep

Much of the research on caffeine and sleep has been concerned with removing unwanted sleepiness either when the person is working at night or when they are sleep deprived. The fact that caffeine can remove sleepiness means that it can, of course, interfere with normal sleep. However, patterns of consumption suggest that individuals usually control their caffeine intake to prevent interference with sleep. If large amounts of caffeine are consumed shortly before trying to sleep, then it will

Conclusions

The present article has reviewed the effects of caffeine on mood, mental performance and sleep. In all areas it is apparent that there is a big difference between the effects of amounts of caffeine that are normally consumed and those observed when excessive amounts are ingested, or when very sensitive individuals are studied. Most of the research has examined acute effects of single doses, and further studies are needed to produce a more detailed profile of effects of regular levels of

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