To assess the alerting effects of caffeine after normal and restricted nocturnal sleep, 36 healthy, nonsmoking men, 19 to 35 years old, who reported normal sleep and daytime alertness received 0, 75, or 150 mg caffeine twice daily after 8 and 5 hours in bed the previous night. Sleep restriction reduced average daily sleep latency measured by the Multiple Sleep Latency Test (MSLT) and slowed auditory vigilance reaction time in the latter half of the 40-minute task. Caffeine (75 and 150 mg) increased average daily sleep latency and improved vigilance reaction time. However, sleep restriction did not alter the alerting effects of caffeine. The data show that, unlike ethanol, basal level of sleepiness/alertness does not interact with the effects of caffeine.