The alerting effects of caffeine were assessed using a standard physiological measure of daytime sleepiness/alertness, the Multiple Sleep Latency Test (MSLT). Healthy young men (n = 24) were randomly assigned to receive caffeine 250 mg or placebo administered double blind, at 0900 and 1300 hours on each of 2 days. On the 3rd day both groups received placebo to test for conditioning to the alerting effects of caffeine. Each day sleep latency was measured at 1000, 1200, 1400, and 1600 hours and performance (divided attention at 1030 hours and auditory vigilance at 1430 hours) was assessed. Caffeine increased sleep latency (i.e., improved alertness) and auditory vigilance performance compared to placebo. Tolerance to the effects of caffeine on sleep latency developed over the four administrations. On the conditioning test (day 3) the group receiving caffeine the previous two days was more alert and performed better than the placebo group.