Well there are many negative effects. BUT there are GOOD effects. Caffeine is a stimulant, meaning it accelerates physiological activity. Specifically, it speeds up the action of your brain and makes you more alert. It does this by binding to adenosine receptors in the brain. Normally the chemical adenosine binds to these, causing drowsiness by slowing down nerve cell activity. The caffeine doesn t have this effect, but does get in the way of the adenosine. Because the caffeine is blocking the adenosine receptors, your neurons become more active than they otherwise would be. That is why it sees to be good for the brain. Then your pituitary gland responds to all the activity as though it was an emergency, by releasing hormones that tell the adrenal glands to produce adrenaline. This is what is sometimes known as the fight or flight hormone (and is also called epinephrine). This release of adrenaline causes: * A faster heart rate. * An opening up of breathing tubes. * A release of sugar into the bloodstream from the liver. * A readying (tightening) of muscles for action. * An increase in blood flow to muscles. Essentially all that adrenaline makes you tensed and ready for action, but not necessarily intellectual action. You do become more alert initially, and your brain may work better and faster. But by the time you start that second cup, you may be anxious and irritable, which is not conducive to clear thinking. Is there a balance that works? Studies which demonstrate an improvement in mental function usually show it after one eight-ounce cup of coffee. Other studies show that drinking two cups (versus none) causes irritability and an increase of the heart rate by 15 to 20 beats-per-minute. So if you must use caffeine, try to limit it to one cup. Tea has less than half the caffeine of coffee, so you may want to switch if you want to enjoy more to drink without getting that racing heart. Some self-experimentation is probably called for, especially since we don t all react in the same way to caffeine. But there is also more to the story. BAD EFFECTS: Other effects of caffeine on the brain include an increase in dopamine levels, much like that caused by amphetamines or heroine (but without such a pronounced effect). Dopamine is a neurotransmitter which activates the pleasure centers of the brain. It is suspected that this is part of the reason caffeine can be so addicting. Alert and happy? You can see why your body and brain likes the stuff. The problem, though, is that long term it can have some pretty nasty effects. For example, once the adrenaline wears off, you can feel depressed and tired. This causes you to crave more caffeine, of course. The problem with that is that it interferes with proper sleep. It is estimated that the half-life of caffeine in your body is about six hours. This means that if you have a large cup of coffee (12 ounces) at four in the afternoon, with about 150 milligrams of caffeine, and then go to bed at ten, you still have about 75 milligrams of caffeine in your system. That may not keep you awake. In fact, some people can sleep with several hundred milligrams of caffeine in their bodies. But that blocking of adenosine receptors will prevent deep sleep, which you need. As a result you are tired the next day and so you reach for more coffee, starting a downward spiral of addiction. Start the cycle and you find it tough to stop.