Dozens of studies have probed the relationship between sleep and memory, with clear evidence that body's natural sleep-wake cycle plays an important role.

The team from the University of Dusseldorf wanted to see just how short a sleep could have any discernable impact.

They used a group of students who were asked to remember a set of words, then given an hour's break before testing.

During that hour, some of the students were allowed to sleep for approximately six minutes, while the rest were kept asleep.

Remarkably, on waking, the napping students performed better in the memory test.

Some theories suggests that the processing of memories takes place in deep sleep, a phase which does not normally start until at least 20 minutes after falling asleep.

However, the team, led by Dr Olaf Lahl, said that it was possible that the moment of falling asleep triggered a process in the brain that continued regardless of how long the person actually stayed awake.

"To our knowledge, this demonstrates for the first time that an ultra-brief sleep episode provides an effective memory enhancement," he wrote.

Professor Jim Horne, from the Sleep Research Centre at Loughborough University, said that while the study was "interesting", he was yet to be convinced that the effect was purely one of memory enhancement.

"The idea that memory could be enhanced in just six minutes is a quite unique finding and one has to be rather cautious about it.

"There is quite a bit of evidence that memory processing probably takes place more than six minutes into sleep."

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