Understanding a little bit more about how caffeine works may also shed some light on why, if you've been drinking coffee for years, you feel generally more run- down and apathetic. Like all toxic substances, caffeine does not affect only one area of the body. Another result of its reapportioning of your body's available energy is that your body requires an increased level of vitamins and minerals to make this happen. This above-average usage can take its toll on your body in the long term.
Caffeine also has a habit of depleting your body's supply of B vitamins, one of your natural sources of energy. So as soon as your "liquid energy" runs out, your body's lack of B vitamins leaves you feeling lethargic and unmotivated. In addition to your B Vitamins, Caffeine encourages the body to dump other key nutrients such as Calcium, Iron, Magnesium, and Potassium, and inhibits the uptake of many others.
The real kicker comes with this: if your body has become tolerant of caffeine, your daily cup of joe (or 10) is not giving you any additional energy.