Many say their brain doesn't start working properly in the morning until they have had a cup of coffee.
But the stimulating effect for which caffeine is famous may be an illusion, say researchers.
Those who drink a lot of coffee may feel more alert after their first cup of the day.
But this is probably because it reverses the fatiguing effects of overnight caffeine withdrawal, say the researchers.
Their study suggests coffee drinkers may actually be better off without their habitual morning mug as it raises the risk of anxiety and high blood pressure.
In the study, 379 people who abstained from caffeine for 16 hours before drinking either caffeine or a placebo (dummy drink) were tested for a range of responses afterwards.
The Bristol University researchers found little variance in levels of alertness among the volunteers, says a report published online in the journal Neuropsychopharmacology.
Peter Rogers, from the university's department of experimental psychology and one of the lead authors of the study, said: 'Our study shows that we don't gain an advantage from consuming caffeine - 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.'
Around half the volunteers drank little or no caffeine and the other half were medium-high caffeine consumers. All were asked to rate their levels of anxiety and alertness and whether they had a headache before and after being given the caffeine or the placebo.
They were also asked to carry out a series of computer tasks to test their levels of memory, attentiveness and vigilance. The mediumwho-high caffeine consumers who received the dummy drink reported a fall in alertness and an increase in headaches, neither of which were reported by those who received caffeine.
However, their post-caffeine levels of alertness were no higher than those of no or low-level consumers received a placebo. This suggests caffeine only brings coffee drinkers back up to 'normal'.
The researchers said their findings could also apply to those who say they rely on a morning cup of tea to get their brains going.
They also found a genetic predisposition to anxiety did not deter people from drinking coffee.
In fact, those with the gene variant associated with anxiety tended to consume slightly larger amounts of coffee than those without the variant, suggesting a mild increase in anxiety may be a part of the pleasant buzz caused by caffeine.
A single cup of instant coffee contains 60-100mg of caffeine depending on the strength of the brew; a cup of tea contains 30-100mg of caffeine; and a latte or espresso contains 90-200mg of caffeine.
A 55g chocolate bar contains 40-50mg of caffeine. But Dr Euan Paul, executive director of the British Coffee Association, said further research was needed to look at how the results of this study may affect the wider population.
'There is 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,' he said.
'This effect is not only found with subjects in a low state of alertness such as night- 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.
'Coffee when consumed in moderation, four-five cups per day, is safe. Pregnant women should should be mindful of the advice given by the Food Standards Agency and limit caffeine intake to 200mg per day from all sources.'
相关中文报道：
你有没有过这样的经历，为了提神，一早连喝三四杯咖啡，却更觉疲倦?最新出版的台湾《康健》杂志告诉你，罪魁祸首可能就是咖啡。
哈佛大学最新研究指出，咖啡中的咖啡因能够干扰中枢神经系统中的镇定物质发挥作用，所以喝咖啡让人清醒、兴奋。但即使早上连灌三四杯咖啡，兴奋作用到中午也会消失，会让人再度感觉昏昏欲睡，反而更加疲惫。因此，最好早晨只喝一杯咖啡，下午感觉疲惫时再喝一杯。这样稳定摄入咖啡因，作用更持久。
香港医院药剂师学会副会长崔俊明对此表示认同，咖啡是种见效很快的提神物质，研究发现，一般饮用1至1.5小时后即可见效。但是，一天若摄取超过400毫克咖啡因(约4杯咖啡)，可能影响睡眠，让人更疲倦，还可能引起心律失常、血压上升。
此外，一项登载在美国《应用社会心理学杂志》上的新研究表明，咖啡的提神效果对女性更明显。