Some apps play pink noise to help people sleep. It sounds soft, like rain. Can it make sleep deeper? In a 2013 study in Germany, scientists played short sounds at exactly the right moments of deep-sleep brain waves.

Deep sleep became stronger, and people remembered more words the next morning. But playing noise all night is different. There is not much proof that it helps. It may only cover other sounds.

Pink noise is a soft, rain-like sound in which higher sounds are weaker than lower ones, and many sleep apps claim that it deepens sleep. In a 2013 German study, researchers read the brain waves of 11 young sleepers in real time and played short bursts of pink noise exactly when a slow, deep-sleep wave was rising.

The deep-sleep waves became larger, and the next morning the participants remembered word pairs better. When the sounds were played at the wrong time, there was no such effect. However, this method needs special equipment, and a 2021 review found that the memory benefit across studies was small. Another review concluded that the evidence for playing noise continuously all night is weak.

Pink noise, a rain-like sound in which higher frequencies are progressively weaker, is widely promoted as a way to deepen sleep, but the evidence depends heavily on how it is delivered. In a 2013 study published in Neuron, German researchers monitored the brain activity of 11 young adults in real time and played brief bursts of pink noise precisely as each slow oscillation of deep sleep was rising. This closed-loop stimulation enlarged the slow waves and improved next-morning recall of word pairs, while sounds delivered out of phase had no such effect. This works like pushing a swing at just the right moment. Similar results were later reported in older adults.

However, a 2021 meta-analysis of ten studies involving 177 participants found that the overall memory benefit was small and only marginally significant, and its authors cautioned that commercial devices could not yet be recommended. A separate systematic review also judged the evidence that continuous noise played throughout the night improves sleep to be of very low quality, although steady sound may help mask sudden disturbances such as traffic.