Caffeine withdrawal is recognised as a medial disorder. There have been numerous articles about it. This is from the October 2004 issue of Psychopharmacology on a study done for the National Institute on Drug Abuse.
"Caffeine is the world's most commonly used stimulant, and it's cheap and readily available so people can maintain their use of caffeine quite easily," says Roland Griffiths, Ph.D., professor of psychiatry and neuroscience at Johns Hopkins. "The latest research demonstrates, however, that when people don't get their usual dose they can suffer a range of withdrawal symptoms, including headache, fatigue, difficulty concentrating. They may even feel like they have the flu with nausea and muscle pain."
It goes on to say
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...common withdrawal symptoms: headache; fatigue or drowsiness; dysphoric mood including depression and irritability; difficulty concentrating; and flu-like symptoms of nausea, vomiting and muscle pain or stiffness. In experimental studies, 50 percent of people experienced headache and 13 percent had clinically significant distress or functional impairment -- for example, severe headache and other symptoms incompatible with working. Typically, onset of symptoms occurred 12 to 24 hours after stopping caffeine, with peak intensity between one and two days, and for a duration of two to nine days. In general, the incidence or severity of symptoms increased with increases in daily dose, but abstinence from doses as low as 100 milligrams per day, or about one small cup of coffee, also produced symptoms...
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..caffeine is the most widely used behaviorally active drug in the world. In North America, 80 percent to 90 percent of adults report regular use of caffeine. Average daily intake of caffeine among caffeine consumers in the United States is about 280 milligrams, or about one to two mugs of coffee or three to five bottles of soft drink, with higher intakes estimated in some European countries. In the United States, coffee and soft drinks are the most common sources of caffeine, with almost half of caffeine consumers ingesting caffeine from multiple sources, including tea.
I avoid caffeine as far as possible, I've found it can upset my stomach if I have too much.