Caffeine is usually consumed in the form of coffee, tea, caffeinated sodas and energy drinks with the hope that it will increase your cognitive functioning and improve your mood. Judging by the length of the waiting lines in Starbucks, this must be especially true in the mornings.
Many of us feel like we really can't function until we get our first cup (or two) of coffee, but how much is too much? It's generally agreed that consuming up to 300 mg of caffeine per day is safe. That's roughly the amount of caffeine you would get from three cups (not mugs or big paper cups) of coffee. Women who are pregnant or may become pregnant may want to decrease that amount or skip the caffeine altogether.
What Caffeine Does
Caffeine is a stimulant and some studies show that small amounts of caffeine may improve your mental response time. Other studies show that the cognitive improvements and mood elevation may not really be due to the beneficial aspects of caffeine as much as ending the withdrawal symptoms you feel when you haven't had your morning "fix" yet.
Too Much Caffeine
Consuming more than 300 mg caffeine per day may give you the "caffeine jitters." Larger amounts of caffeine may make you irritable, sleepless, and may even trigger anxiety and cause diarrhea.
It was assumed for a long time that drinking too much coffee or other caffeinated beverages would cause dehydration because caffeine can act as a diuretic. However, researchers found that your body adjusts to your caffeine intake so drinking caffeinated beverages won't increase your need for water.
Kicking the caffeine habit cold turkey isn't so good. Caffeine withdrawal can give you headaches, make you crabby, give you muscle aches and generally make you feel miserable for a few days, but after a week or so, the withdrawal symptoms will pass.
Sources:Armstrong LE, Pumerantz AC, Roti MW, Judelson DA, Watson G, Dias JC, Sokmen B, Casa DJ, Maresh CM, Lieberman H, Kellogg M. "Fluid, electrolyte, and renal indices of hydration during 11 days of controlled caffeine consumption." Int J Sport Nutr Exerc Metab. 2005 Jun;15(3):252-65.
Rogers PJ, Dernoncourt C. "Regular caffeine consumption: a balance of adverse and beneficial effects for mood and psychomotor performance." Pharmacol Biochem Behav. 1998 Apr;59(4):1039-45.
Smith AP., "Caffeine at Work." Hum Psychopharmacol. 2005 Aug;20(6):441.