Caffeine, Caffeine Withdrawal and Psychomotor Performance: A Reply to James

Caffeine Caffeine withdrawal Psychomotor peformance Andrew P. Smith, Health Psychology Research Unit, Department of Psychology, University of Bristol, 8 Woodland Road, Bristol, BS8 1TN (UK) James [1] has questioned whether the superior performance and increased alertness found in caffeine conditions in a recent series of studies [2-5] were due to actual enhancement by caffeine or merely reflected performance and alertness being degraded by caffeine withdrawal in the two caffeine-free conditions. The present letter to the editor addresses this issue and considers a number of pieces of evidence which produce problems for the ‘caffeine withdrawal’ explanation of beneficial behavioural effects of caffeine. The view that beneficial effects of caffeine reflect degraded performance and alertness in the caffeine-free conditions crucially depends on the strength of the evidence for withdrawal effects. James states that ‘there is an extensive literature showing that caffeine withdrawal has significant adverse effects on human performance and well-being’. If one examines the details of the studies James cites to support this view [6-10] one finds that effects of withdrawal were selective, influencing some functions only, and were not very pronounced. Indeed, these studies provide no evidence that the functions examined in our research are degraded by caffeine withdrawal. Furthermore, even most recent studies of the effects of caffeine withdrawal on performance and well-being [11] can be criticised on methodological grounds (e.g. failing to consider the importance of order of caffeine/placebo conditions). A second point made by James himself in an earlier article [ 12] is that the effects of caffeine on performance and mood are variable and influenced by contextual factors. If withdrawal was the major factor in these studies one should find that, provided enough caffeine was given to prevent withdrawal, further increases in dose should have no effect. This is clearly not the case [13]. Another problem for the ‘caffeine withdrawal’ explanation is that it cannot account for effects in naive users or in animals that have never had caffeine before. Yet behavioural effects clearly occur in these groups and one might expect them to be larger than those found with regular users, where some habituation may take place. James rejects such data as reflecting only ‘a small minority of the population’. Three results from our recent studies also cause problems for the ‘caffeine withdrawal’ explanation. First, it is possible to demonstrate the same effects of caffeine on performance when a person has abstained for only 1 h [14] as when they have had no caffeine for over 12 h [15]. Secondly, level of regular caffeine usage is not correlated with changes in alertness