Now, writing in the journal Nature Communications, a team of researchers from Charité—Universitätsmedizin Berlin posits an explanation. According to the study, the slow waves make the neocortex, the location of long-term memory, especially receptive to information. The findings could help to optimize the treatment approaches that are intended to support memory formation from outside.

How do permanent memories form? Experts believe that while we sleep, our brains replay the events of the day, moving information from the location of short-term memory, the hippocampus, to the long-term memory located in the neocortex.

"Slow waves" are especially key to this process: slow, synchronous oscillations of electrical voltage in the cortex that occur during the deep sleep phase. They can be measured using an electroencephalogram (EEG). The waves originate when the electrical voltage in many neurons rises and falls simultaneously once per second.

"We've known for many years that these voltage fluctuations contribute to the formation of memory," explains Prof. Jörg Geiger, director of the Institute of Neurophysiology at Charité and the head of the newly published study.