 Studies have demonstrated that people who take in a minimum of 100mg of caffeine per day (about the amount in one cup of coffee) can acquire a physical dependence that would trigger withdrawal symptoms that include headaches, muscle pain and stiffness, lethargy, nausea, vomiting, depressed mood, and marked irritability.Professor Roland R. Griffiths, a professor of neurology at Johns Hopkins in Baltimore strongly believes that caffeine withdrawal should be classified as a psychological disorder.His research suggested that withdrawal affects 50% of habitual coffee drinkers, beginning within 12–24 hours after cessation of caffeine intake, and peaking in 20–48 hours, lasting as long as 9 days.