Spanish researchers said caffeine worked faster on men to perk up their energy levels than it did on women. Researchers at the University of Barcelona studied the effects of coffee consumption on 668 university students measuring activity levels and sleepiness before drinking a coffee and up to 30 minutes after.

The study subjects were asked to drink a classic espresso which contained 100 milligrams of coffee or a decaffeinated espresso which contained only 5 milligrams of caffeine. The researchers noted that the men were re-energized within 10 minutes of drinking the coffee, while the women were also alert but not as much as the men.

Study lead author Ana Adan, a researcher in the university's psychiatry and clinical psychobiology department said, "We observed a greater impact among the males. If a person cannot drink normal coffee, a decaffeinated one might provide some benefits."

Earlier studies had shown the stimulating effects of caffeine but very few had taken the gender question into account. "Numerous studies have demonstrated the stimulant effects of caffeine, but none of these have looked at their effects in terms of the consumer's gender," Adan said.

The researchers said the caffeine kick begins within 45 minutes of drinking it and lasts for two to three hours. The time the energy boost lasted varied from person to person depending on factors like age, individual sensitivity, metabolic rate etc. They added that further studies were needed to establish whether the energy boost from decaffeinated coffee was real or due to a placebo effect.

The study, which is published in the journal Progress in Neuro-Psychopharmacology & Biological Psychiatry, said this kind of research could one day lead to dietary recommendations based on genetic make-up, body size and age.