Effects of caffeine, time of day and user history on study-related performance.

The individual and interactive effects of caffeine, time of day and history of caffeine consumption on several study-related tasks were investigated in 25 subjects (6 males, 19 females). Performance was measured on short term memory (STM), mental arithmetic (MA), reading comprehension, serial search (SS) and verbal reasoning (VR). Subjects attended eight experimental sessions, at four times of day (0100, 0700, 1300 and 1900 hours), after ingesting caffeine (4 mg/kg) or placebo. Subjects were assigned to a low, moderate or high user group on the basis of a caffeine consumption questionnaire. Reading comprehension was affected by time of day, while caffeine improved performance on all mental speed-related tasks. High caffeine users performed more poorly than other groups on the verbal reasoning task. Several interactions between the three independent variables were observed on a number of tasks, supporting the contention that different processes underlying various types of cognitive performance are differentially, and often jointly, affected by caffeine, time of day and user history. Implications of caffeine usage on academic performance were discussed.