How sleep helps us consolidate memories is still largely a mystery. A recent study from the
University of Lübeck, in Germany, offers one clue. Subjects were given a list of 46 word pairs to
memorize, just before sleep. Shortly after they fell asleep, as they reached the deepest stages of
sleep, electrical currents were sent through electrodes on their heads to induce very slow brain
waves. Such slow waves were induced at random in the brains of one group of subjects, but not
another. The next morning, the slow-wave group had better recall of the words. Other types of memory were not improved, and inducing the slow waves later in the night did not have the same
effect. Why and how the slow waves improved memory is not yet understood, but they are
thought to alter the strengths of chemical connections, or synapses, between specific pairs of
nerve cells in the brain. Memories are "stored" in these synapses: changing the strength of the
synapses increases the strength of the memories they store.