Effects of caffeine on human behavior
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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