Caffeine is a central nervous system and metabolic stimulant,and is used both recreationally and medically to reduce physical fatigue and to restore alertness when drowsiness occurs. It produces increased wakefulness, faster and clearer flow of thought, increased focus, and better general body coordination.The amount of caffeine needed to produce effects varies from person to person, depending on body size and degree of tolerance. Effects begin less than an hour after consumption, and a moderate dose usually wears off in about five hours.Caffeine has a number of effects on sleep, but does not affect all people in the same way. It improves performance during sleep deprivation but may lead to subsequent insomnia.In shift workers it leads to fewer mistakes caused by tiredness.In athletics, moderate doses of caffeine can improve sprint, endurance,and team sports performance,but the improvements are usually not very large. High doses of caffeine, however, can impair athletic performance by interfering with coordination.Evidence shows that, contrary to common advice, caffeine may be helpful at high altitude.Physical effects
Consumption of large amounts of caffeine — usually more than 250 mg per day — can lead to a condition known as caffeinism. Caffeinism usually combines caffeine dependency with a wide range of unpleasant physical and mental conditions including nervousness, irritability, restlessness, insomnia, headaches, and heart palpitations after caffeine use.
Caffeine can have both positive and negative effects on anxiety disorders depending on the dose. At high doses, typically greater than 300 mg, it can both cause and worsen anxiety.