In a recent study on the cognitive effects of sleep, researchers found that volunteers who napped for 90 minutes during the day performed better on cognitive tests than those who were kept awake. It has been suggested that sleeping allows the brain to process short-term memories, moving them from the hippocampus(海馬)to the long-term storage of the cortex(皮質、外皮). The study's lead author likens the process to an email inbox that is full and must be cleaned out in order to receive new information. This "filing" appears to occur during stage 2 non-rapid eye movement sleep.