Caffeine is an alkaloid found in both the seeds and leaves of coffee, tea, cacao, cola and mate plants. A cup of coffee contains about 50 mg of caffeine and its stimulant action, which lasts for one or two hours after it has been drunk, acts on the cerebrospinal nervous system, heightening our mental faculties, keeping away sleepiness, boredom, fatigue (including mental fatigue) and depression. It bolsters memory, learning capacity, perception and concentration, and facilitates the perception of sensory stimuli; it also abates headaches and migraines.
The scientific community agrees on the amount of caffeine that can be consumed in a day, indicating 300 mg/day of caffeine for an adult, i.e. about 4-5 mg of caffeine per kg of body weight per day: e.g.:
a man weighing 70 kg (154 lb) = 4.5 x 70 = 315 mg.
To get an idea of what it means in terms of dose, these are the quantities of caffeine contained in some of the most common beverages/foods:
For more information: Coffee and health