Sex differences in estimation of time intervals and in reaction time are removed by moderate but not high doses of caffeine in coffee

Estimation of the passage of time in the seconds-to-minutes range and react ion time are strongly dependent on a hypothetical internal clock. Dopamine is the neurotransmitter most closely related to the rate of this clock. Caf feine, probably the most consumed drug in the world, leads to an augmentati on of dopamine neurotransmission. In this study coffee, which reproduces th e conditions under which caffeine is normally ingested, containing 3, 75, 1 50 or 300 mg of caffeine, was given to healthy male and female volunteers. A computerized time estimation and reaction time test was carried out 50 mi n after ingestion. Sex differences in placebo control subjects (who took de caffeinated coffee with 3 mg of caffeine), with females making more accurat e estimates of time intervals than males and males showing shorter reaction times than females, were removed in subject taking doses of 75 and 150 mg of caffeine in the case of time estimation and 150 mg in the case of reacti on time. The 300 mg dose induced overestimation of time in females and shor tened the reaction time in males. There were no sex differences in the phar macokinetics of caffeine, as measured in salivary concentration of caffeine using high-performance liquid chromatography. Results indicating sex diffe rences in time estimation or reaction time should not be generalized from t he laboratory to real life without considering the fact that everyday coffe e consumption may eliminate these differences. Copyright (C) 2001 John Wile y & Sons, Ltd.