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.'

相關(guān)中文報(bào)道：

你有沒有過這樣的經(jīng)歷，為了提神，一早連喝三四杯咖啡，卻更覺疲倦?最新出版的臺(tái)灣《康健》雜志告訴你，罪魁禍?zhǔn)卓赡芫褪强Х取?/p>

哈佛大學(xué)最新研究指出，咖啡中的咖啡因能夠干擾中樞神經(jīng)系統(tǒng)中的鎮(zhèn)定物質(zhì)發(fā)揮作用，所以喝咖啡讓人清醒、興奮。但即使早上連灌三四杯咖啡，興奮作用到中午也會(huì)消失，會(huì)讓人再度感覺昏昏欲睡，反而更加疲憊。因此，最好早晨只喝一杯咖啡，下午感覺疲憊時(shí)再喝一杯。這樣穩(wěn)定攝入咖啡因，作用更持久。

香港醫(yī)院藥劑師學(xué)會(huì)副會(huì)長(zhǎng)崔俊明對(duì)此表示認(rèn)同，咖啡是種見效很快的提神物質(zhì)，研究發(fā)現(xiàn)，一般飲用1至1.5小時(shí)后即可見效。但是，一天若攝取超過400毫克咖啡因(約4杯咖啡)，可能影響睡眠，讓人更疲倦，還可能引起心律失常、血壓上升。

此外，一項(xiàng)登載在美國(guó)《應(yīng)用社會(huì)心理學(xué)雜志》上的新研究表明，咖啡的提神效果對(duì)女性更明顯。