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Research subjects spent two nights at a sleep lab, with auditory stimulation – low-volume bursts – synchronized to their brain waves.

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For almost a century, scientists have known that sleep plays a critical role in the creation of long-term memories. What we experience during the day is processed and stored away in our brains as we sleep.

Now, German researchers say they have found a relatively simple way to boost memory formation by playing sounds at certain times during the sleep cycle. If they are right, the discovery could be a godsend to students cramming for exams or anyone who wants to improve their recall.

Previous studies have shown that memory consolidation occurs when brain waves are generating a slow oscillating rhythm, said one of the researchers, Jan Born of the University of Tubingen. The new study, published Thursday in the journal Neuron, reveals that if a person is exposed to an auditory stimulation that is in sync with this rhythm, the oscillations become amplified and last longer – and memory is improved.

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For the study, the researchers recruited 11 healthy students who spent two nights at a sleep lab. On both occasions, the participants were asked to memorize a list of word associations (120 word pairs like brain/consciousness and solution/problem) before going to bed. Electrodes were attached to their scalps so their brain waves could be observed as they slept.

On one of the nights, the volunteers were exposed to auditory stimulation synchronized to their brain waves. On the other night, sleepers got sham stimulation that was out of phase with the brain's slow oscillation rhythm.

"We saw the same effect in all subjects," Born said. "Memory improved with auditory stimulation," but not with the sham therapy. "And the effect was statistically very robust."

Born said the stimulation works in the same way as a child's swing set. "If you push the swing at the right phase, the swing will increase in amplitude," he explained. "You don't need to use a lot of energy. It's all about the timing."

In a similar fashion, the auditory stimulation was fairly subdued. It consisted of a series of low-volume bursts, each one lasting a fraction of a second. Only a few of the subjects had any recall of hearing something out of the ordinary during the night.

To get their timing right, the researchers closely monitored the data picked up by the scalp electrodes. "Slow oscillations occur in kind of a train of several oscillations succeeding one another," he said. "When one slow oscillation was detected, it triggered the computer to deliver an auditory stimulus at exactly the time … the next one was expected."

Several years ago, the German researchers carried out a similar experiment, but used electric current instead of sound. During that study, the electrical stimulation wasn't co-coordinated with brain waves.

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"The charm of the auditory approach, compared to electrical stimulation, is that it is less invasive and it is very easy to achieve the results. The brain itself is the pacesetter" for determining when to deliver the stimulus.

Although the study was conducted in a sleep lab and the participants had to wear bulky electrodes, Born believes it's possible to create compact equipment suitable for use in a home. But the researchers have no plans to commercialize their discovery. "We are basic scientists," said Born. "If someone wants to make money out of this, he should feel free to make such [home] devices."

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