Many swear by their daily cup of java to kick-start their brains in the morning or increase alertness in the afternoons. But they may actually be kidding themselves if they think that coffee helps. Caffeine addiction is such a downer that regular coffee drinkers may get no real pick-me-up from their morning cup, according to a study by British scientists, which appeared recently in the Neuropsychopharmacology journal.

Bristol University researchers found that drinkers develop a tolerance to both the anxiety-producing and the stimulating effects of caffeine, meaning that it only brings them back to baseline levels of alertness, not above them.

"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," wrote the scientists, led by Peter Rogers of Bristol's department of experimental psychology.

The team asked 379 adults - half of them non- or low-caffeine consumers and the other half medium- or high- caffeine consumers - to give up caffeine for 16 hours, and then gave them either caffeine or a dummy pill known as a placebo.

Participants rated their levels of anxiety, alertness and headache. The medium-to-high caffeine consumers who got the placebo reported a decrease in alertness and increased headache, neither of which were reported by those who received caffeine. But measurements showed that their post-caffeine levels of alertness were actually no higher than the non- or low-caffeine consumers who received a placebo - suggesting caffeine only brings coffee drinkers back up to "normal."

The researchers also found that people who have a genetic predisposition to anxiety do not tend to avoid coffee. In fact, people in the study with a gene variant associated with anxiety tended to consume slightly larger amounts of coffee than those without it. This suggests that a mild increase in anxiety "may be a part of the pleasant buzz caused by caffeine," said Rogers.