LONDON: Stimulating the brain with gentle electric currents during sleep boosts memory, German scientists said on Sunday.

When they applied several currents that mimic natural slow oscillating brain waves in sleep they enhanced the memory of medical students who had done a word-learning task.

"It leads to improved memory retention," said Jan Born, a neuroscientist at the University of Luebeck. The scientists, whose results were published online by the journal Nature, believe brain stimulation could help people with memory problems and Alzheimer's disease.

"This is an alternative way to intensify or to improve sleep and its memory function," Born said. He and his team asked the students to learn a list of paired words in a standard memory test before they fell asleep. The researchers stimulated their brain while they slept. After they woke up, the students had to recall the words they had memorised.

If the currents were applied to the scalp during deep sleep, the first few hours of nocturnal sleep, the students recalled a greater number of words than if they had been given a sham brain stimulation.

"This is proof that this slow oscillation has a real function during sleep to build and consolidate memory," said Born. "It is an eight percent increase overall. This is a striking increase," he added.

The students did not feel any sensation from the currents to the frontal cortex of the brain or any adverse side effects. The currents forced the brain more into the deep slow-wave sleep to improve the memory function, according to the scientists. Memory function in the medical students was already very good before they received the brain stimulation but the currents managed to improve it.

"There is growing evidence that you can very effectively manipulate brain function by different types of electrical simulation," Born said. He believes the natural slow oscillations and those induced by the electrical currents affect the hippocampus area of the brain which plays a part in memory.

"The slow oscillations during slow-wave sleep trigger a kind of replay of these memories in the hippocampus," he added. The hippocampus is one of the first regions of the brain that is damaged in patients with Alzheimer's disease, a degenerative illness that robs people of their memory and cognitive ability.