A double espresso shot after swotting for an exam boosts the memory, according to US scientists.

A study of 100 people claims to have the first clear evidence of caffeine's memory-boosting effect, lasting at least 24 hours.

Volunteers took part in a double-blind trial in which they were either given a 200 milligram caffeine pill or dummy placebo tablet five minutes after studying a series of images.

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Tests a day later proved that the memory of those who took caffeine had been enhanced at a deep level.

The amount of caffeine used was equivalent to a double strong espresso.

Lead researcher Dr Michael Yassa, assistant professor of psychological and brain sciences at Johns Hopkins University in Baltimore, said: "We've always known that caffeine has cognitive-enhancing effects, but its particular effects on strengthening memories and making them resistant to forgetting has never been examined in detail."