AbstractThe stimulant modafinil has proved to be an effective treatment modality for narcolepsy and related sleep disorders and is also being studied for use during sustained military operations to ameliorate the effects of fatigue due to sleep loss. However, a previous study reported that a relatively large, single dose of modafinil (300 mg), administered to already sleep‐deprived individuals, caused participants to overestimate their cognitive abilities (i.e. ‘overconfidence’). Because the predominant application of modafinil is in otherwise healthy, non‐sleep‐deprived individuals, the present study investigated the generality of modafinil‐induced overconfidence in a group of 18 healthy, non sleep‐deprived adults. The design involved a double‐blind, placebo controlled, fully within‐subjects manipulation of placebo and modafinil (4 mg/kg: approximately 300 mg, on average) over three 50‐min cognitive testing sessions (i.e. before drug ingestion, and at 90 and 180 min after drug ingestion). The cognitive task battery included subjective assessments of mood, fatigue, affect, vigor and motivation, and cognitive assessments of serial reaction time, logical reasoning, visual comparison, mental addition and vigilance. In addition, trial‐by‐trial confidence judgements were obtained for two of the cognitive tasks and more global, task level assessments of performance were obtained for four of the cognitive tasks. Relative to placebo, modafinil improved fatigue levels, motivation, reaction time and vigilance. In terms of self‐assessments of cognitive performance, both the placebo and modafinil conditions were ‘well calibrated’ on trial‐by‐trial confidence judgements, showing neither marked over‐ nor under‐confidence. Of note, the modafinil condition displayed a non‐significant tendency towards ‘overconfidence’ for task‐level assessments of performance. The present findings highlight the need for continued research on the many complex interactions involving fatigue states, occasional versus long‐term stimulant use, and subjective assessments of fatigue and cognitive performance. Copyright © 2004 John Wiley & Sons, Ltd.