Caffeine, while famous for its ability to conjure and sustain mental alertness, this odorless, bitter alkaloid has more effects then many are aware of. All of caffeine's effects are subject to individual differences in how one's body responds to caffeine and all bodies respond differently. Caffeine sensitivity is affected by body weight and build, tolerance, metabolism, hormonal status, whether he/she smokes or takes medication, and liver function. The onset of caffeine's effects occur 5 - 45 minutes after ingestion, and typically last one and a half to five hours.
A good summary of the effects appears in The Pharmacological Basis of Therapeutics by Dr. J. Murdoch Ritchie: it reads caffeine produces " a more rapid and clearer flow of thought....After taking caffeine one is capable of greater sustained intellectual effort and a more perfect association of ideas. There is also a keener appreciation of sensory stimuli, and motor activity is increased; typists, for example, work faster and with fewer errors." Additionally, heart rate increases, respiration increases, metabolic rate increases, blood vessels dilate, stomach acid and urine are produced, smooth muscles are relaxed, and movement of fluid and solid wastes from the body are promoted. Such effects are after about one or two cups of coffee (around 200mg caffeine). These effects generally increase as one's caffeine consumption increases, however, the sleep deprivation that caffeine may led to has its own effects which may result in one being mentally rapid and alert, but with little clarity and focus. If large doses of caffeine are taken (around 10 cups of coffee drunk quickly, 1gram caffeine ), mental confusion, headache, chills, fever, irregular heartbeats, and vomiting may result. Caffeine is estimated to be lethal around 4 to 10 grams).
Many have their own idiosyncratic side effects of caffeine due to its primary effects. For example, as stated above, caffeine relaxes smooth muscles and increases stomach acid production. In some people caffeine relaxes the sphincter muscles at the top of the stomach enough to allow the now increased level of stomach acid to backup into their throat causing heartburn. Many view caffeine much like a tool for increased productivity, creativity, or wit. Here are a few quotes extolling the effects of caffeine.
Without my morning coffee I'm just like a dried up piece of roast goat. -Johann Sebastian Bach
For fifteen days I struggled to prove that no functions analogous to those I have since called Fuchsian functions could exist; I was then very ignorant. Every day I sat down at my work table where I spent an hour or two; I tried a great number of combinations and arrived at no result. One evening, contrary to my custom, I took black coffee; I could not go to sleep; ideas swarmed up in clouds; I sensed them clashing until, to put it so, a pair would hook together to form a stable combination. By morning I had established the existence of a class of Fuchsian functions, those derived from the hypergeometric series. I had only to write up the results which took me a few hours. - Henri Poincare,
"Science et Methode": Coffee: creative lighter fluid. --Floyd Maxwell (1957-)