Coffee addicts might insist they can't function without their morning cup of "joe," but new research suggests that true addicts actually get no extra boost from caffeine.

Researchers from Bristol University in Britain have found that the stimulant effect of caffeine is nothing more than an illusion. Their study found that the groggy feeling that many coffee addicts feel in the morning is really acute caffeine withdrawal. That means when addicts finally do get their coffee "fix," the most it can do for them is to bring them back to baseline levels of alertness.

The study, published online in the journal Neuropsychopharmacology, finds that frequent coffee drinkers develop a tolerance to both the anxiety-producing effects and the stimulatory effects of caffeine.

The researchers, led by Peter Rogers of Bristol's department of experimental psychology, came to their conclusions after testing 379 people who abstained from caffeine for 16 hours. Half were non- or low caffeine consumers and the other half were medium or high caffeine consumers. Researchers then randomly gave the volunteers either caffeine or a placebo.

Participants rated their levels of anxiety, alertness and headache. The medium/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.

The volunteers were then asked to carry out a series of tasks on a computer to test their memory, attentiveness and vigilance. The results showed that their post-caffeine levels of alertness were actually no higher than the non/low consumers who received a placebo.

"Our study shows that we don't gain an advantage from consuming caffeine," Rogers said in a news release. "Although we feel alerted by it, this is caffeine just bringing us back to normal.

"On the other hand, while caffeine can increase anxiety, tolerance means that for most caffeine consumers this effect is negligible."

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," he said.