Clicking sounds timed to brainwaves boosted sleep memory

Researchers at the University of Tübingen played soft clicking sounds through sleeping adults' headphones, timed to arrive exactly in step with the brain's own slow oscillations, the under-one-hertz rhythm that marks the deepest stage of sleep.

Clicks delivered in phase with the rising part of each slow wave amplified the wave itself and the fast spindle activity riding on top of it, and people recalled more of the word pairs they had studied the evening before. The identical clicks, delivered slightly out of phase, did nothing.

The timing mattered more than the sound itself: the same clicks, mistimed by a fraction of a second, provided no benefit, which pointed to the brain's own rhythm as the active ingredient rather than any general effect of noise during sleep. The method has since been tested as a tool for aging brains, where slow oscillations fade first.

Timing turned sound into memory.