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This chapter begins with a brief discussion of caffeine metabolism and mechanism of action. The half-life of caffeine in the circulatory system varies substantially between individuals, and is influenced by health, lifestyle, and genetic factors. In cigarette smokers, caffeine's half-life is approximately 3 hours. Caffeine is metabolized in the liver by a complex series of reactions. Genetic variation accounts for some inter-individual differences in caffeine metabolism. Caffeine is structurally similar to adenosine, an inhibitor of neuronal activity in the central nervous system (CNS) with sedative-like properties. Under normal physiological conditions, the behavioral and ergogenic effects of caffeine are due to competitive antagonism at central adenosine receptor sites. Caffeine's effects during sustained marksmanship tasks in rested personnel have been investigated in both laboratory and field studies. The effects of caffeine during severe operational stress and sleep deprivation were examined in a field study conducted with US Navy SEAL trainees.