Caffeine is a central nervous system stimulant and is used to reduce physical fatigue and to prevent or treat drowsiness. It produces increased wakefulness, increased focus, and better general body coordination. The amount of caffeine needed to produce these effects varies from person to person, depending on body size and degree of tolerance. Desired effects begin approximately one hour after consumption, and a moderate dose usually subsides in about three or four hours. Caffeine can delay or prevent sleep, and improves task performance during sleep deprivation. Shift workers have fewer mistakes caused by drowsiness. At normal doses, caffeine has variable effects on learning and memory, but it generally improves reaction time, arousal, and concentration.