Caffeine is the most widely consumed behavior-modifying chemical in the world. It is an alkaloid that interferes with a particular signaling system used by many different cells, and therefore has several different effects on the human body. Above all, caffeine stimulates the central nervous system, relieves drowsiness and fatigue, and quickens reaction times. It also increases energy production in muscles and so their capacity for work. It’s said to improve mood and mental performance, though recent studies suggest that these may be the result of relieving the initial symptoms of overnight caffeine withdrawal! Less desirably, in high doses it causes restlessness, nervousness, and insomnia. It has complex effects on the heart and arteries, and can produce an abnormally fast heartbeat. There is some evidence that caffeine speeds the loss of calcium from bone, so habitual consumption may contribute to osteoporosis.