Contrary to popular belief, darker roasted beans actually have less caffeine. The heat literally roasts out the caffeine. Caffeine acts as a stimulant on the central nervous system. That means it increases the heart rate and blood pressure. The individual may temporarily feel more awake and energetic.
Caffeine also acts as a diuretic, which means it works on the kidneys to excrete more urine.
Someone who wants to stop drinking caffeine should start by mixing regular coffee with a decaffeinated version. It's helpful to gradually reduce the amount of caffeine ingested each day. Caffeine is one of the most thoroughly studied food constituents. Here are some recent research findings. This amount is equal to 3 cups of coffee.