When One Cup Isn't Enough: The Effects of Caffeine Consumption, Tolerance, and Withdrawal on an Individual's Energy Levels. Caffeine is the most popular psychoactive compound in the world, consumed daily by 80% of the world's population (James & Keane, 2007; Rogers, Heatherly, Mullings & Smith, 2013). Caffeine has long been believed to have physiologically enhancing properties and is often used to help increase energy levels (James & Keane, 2007). Drug tolerance is defined as the reduction of the effect of a given dose of a drug to less than the original dose, after repeated administrations (Siegel, 1975). There are multiple theories of tolerance, which will be used in this study to help explain tolerance. affects caffeine. Previous research has attempted to investigate the development of tolerance, discussing the role of environmental cues such as smell, sight, taste, etc. in conditioning theory and the development of caffeine tolerance (Sigel 1975; Poulos, Hinson & Sigel 1981). In contrast, James and Keane (2007) argue that the central cause of drug tolerance is physiological, emphasizing the effects of caffeine on the neuromodulator adenosine. Finally, Peper (2004) theorizes that conditioning theory and physiological theories both contribute to an individual's response to a drug. Few studies have been conducted to investigate the effects of caffeine and caffeine tolerance on organisms (Peper, 2004). Since little is known about such relationships, it is important to develop greater understanding. Higher levels of caffeine consumption are hypothesized to be associated with higher tolerance levels, it was predicted that high caffeine consumers would report smaller changes in energy levels than low level consumers... middle of paper.. ... with the retreat reversal effect and may serve to explain the lack of support for this hypothesis. Another possible explanation for these results is shown in the variance of the population sample size, with significant variance in the number of consumers from low to high. With approximately 80% of the world's population consuming caffeine on a daily basis (James & Keane, 2007), the tolerance levels of the general population are not adequately reflected in the sample sizes chosen. While the limitations may be significant, this study contributed positively to the limited body of research on the tolerance and effects of caffeine consumption. Future experiments may wish to consider a period of overnight abstinence prior to testing to account for systemic levels of caffeine (Rogers et.al, 2013) and also consider the impact of the time of day the experiment is conducted.