Caffeine and time of day effects on a force discrimination task in humans

The effects of caffeine (0 mg/kg, 1 mg/kg, 3 mg/kg) and time of day (TOD) on human performance were studied using a multiple forceband discrimination task (MFDT) and subjective ratings. Self-rated measures of energy level were affected by TOD and caffeine, while mood was affected by TOD. Energy level decreased throughout the day and was offset by caffeine which increased energy level independent of TOD. Self-reported anxiety was not affected by TOD or caffeine. Mood was affected by TOD in a complex cubic trend with late morning and late evening peaks 12 h apart. MFDT performance was affected by TOD, caffeine dosage, and their interaction. Trend analyses showed varying patterns of TOD effects across peak force variability, response latency, response duration, and correct responding. Results support and extend previous findings of TOD influences on the MFDT and support the utility of multicomponent proprioceptive tasks for examining drug effects on performance.