Electrically stimulating the brain while it sleeps can boost memory formation, scientists have found. Writing in this week's Nature, University of Lubeck researcher Jan Bonn describes how he and his team recruited medical students and gave them a list of word-pairs to learn. Wires were attached to the students' heads and when they subsequently slept they received electrical stimulation designed to augment the natural slow wave brain activity seen when a person first nods off. Compared to when the students received "sham stimulation", in other words wore the electrodes but received no stimulation, subsequent recall amongst the stimulated subjects improved significantly. But the effect disappeared if the stimulation was delivered towards the end of a night's sleep. Jan Bonn thinks that the memory-boosting effect occurs because it encourages "replay activity", when a region of the brain called the hippocampus replays things learned during the day so that they can become consolidated into long term memory. Previously researchers had believed that these slow brain waves were an insignificant side effect of brain activity.