Need a pick me up? If you drink too much coffee, you'll stop getting a buzz from caffeine.

379 volunteers stopped drinkiing coffee for 16 hours. Then half of them were given a shot of espresso strength and the rest a caffeine-free coffee. Those with the placebo became headachy and less alert.The ones with the real coffee, however, were not much more alert, once the caffeine had worn off, than those who'd not had the caffeine. The researchers concluded that caffeine in those drinking more than four or five cups of coffee a day only brings them back up to a baseline or normal level. More cups than that, you could feel more drowsy when you stop than if you stick to four or five cups a day.

Said the researchers, "Although frequent consumers feel alerted by caffeine, especially by their morning tea, coffee or other caffeine-containing drink, evidence suggests that this is actually merely the reversal of the fatiguing effects of acute caffeine withdrawal." 

Reaction from the British Coffee Association is that there is what it calls an "overwhelming wealth of evidence" showing that caffeine does increase alertness levels by acting as a stimulant on the central nervous system by prompting the release of adrenaline. "This effect is not only found with subjects in a low state of alertness, like night time shift workers, or those who wake-up early in the morning, but is additionally found in subjects who already have a high state of alertness."

More research needed? Pour yourself another cup...