Caffeine is a stimulant that blocks certain chemicals in the brain, preventing signals of sleepiness and languor from being transmitted. Caffeine can also improve reaction times, memory and reasoning skills. However, in high doses, caffeine can make people anxious or irritable and make it difficult to fall asleep at night.
Caffeine imitates adenosine, a neurochemical that creates feelings of fatigue and tiredness. Caffeine enters the adenosine receptors without actually activating them, preventing those feelings from taking hold and allowing dopamine and serotonin to stimulate the brain freely. Caffeine's temporary energy boosts are associated with increased heart rate, stomach ache and headache.
Caffeine is addictive and can cause withdrawal symptoms of headaches, drowsiness and irritability. Over time, most people develop a tolerance to caffeine and require more to block a similar amount of adenosine. The amount that a person can drink depends on her height, weight, age and metabolism. Withdrawal symptoms can last up to 10 days.
The initial symptoms of caffeine intoxication include restlessness, cramping, nervousness and insomnia. After 1,000 milligrams of caffeine, symptoms may include muscle twitches, rapid heartbeat and psychomotor agitation, among others.
While coffee and tea are the most well-known sources of caffeine, the chemical is commonly found in chocolate, soda and medication. Even coffee-flavored ice creams or snacks contain variable amounts of caffeine.