A sleeping brain can form fresh memories, according to a team of neuroscientists. The researchers played complex sounds to people while they were sleeping, and afterward the sleepers could recognize those sounds when they were awake. The idea that humans can learn while asleep, a concept sometimes called hypnopedia, has a long and odd history. Researchers accomplished pattern learning. While a group of 20 subjects was sleeping, the neuroscientists played clips of white noise. Most of the audio was purely random but there were patterns occasionally embedded within the complex noise: sequences of a single clip of white noise, 200 milliseconds long, repeated five times. The subjects remembered the patterns. The lack of meaning worked in their favor; sleepers can neither focus on what they're hearing nor make explicit connections, the scientist said. This is why nocturnal language tapes don't quite work — the brain needs to register sound and semantics.
But memorizing acoustic patterns like white noise happens automatically. Once the sleepers awoke, the scientists played back the white-noise recordings. The researchers asked the test subjects to identify patterns within the noise. Unless you happened to remember the repetitions from a previous night's sleep. The test subjects successfully detected the patterns far better than random chance would predict. What's more, the scientists discovered that memories of white-noise pattern formed only during certain sleep stages. When the authors played the sounds during REM and light sleep, the test subjects could remember the pattern the next morning. During the deeper non-REM sleep, playing the recording hampered recall. Patterns presented during non-REM sleep led to worse performance, as if there were a negative form of learning. This marked the first time that researchers had evidence for the sleep stages involved in the formation of completely new memories.