Few doubt the benefits of a good night's sleep — and research has long suggested that it helps our memories to consolidate recently learned facts. On page 610, Jan Born and his colleagues at the University of Lübeck in Germany provide fresh insight into how this consolidation process works.

Born and his team focused on the idea that electrical activity might play a role in our memories. Oscillations in electrical activity occur at different rates depending on the phase of the sleep cycle: they are at their peak during rapid-eye-movement (REM) sleep and are at their slowest during periods of deep sleep. “We asked whether these oscillations are critical to the consolidation of memories,” says Born.

Born and his colleagues were aware that there was some kind of relationship between the hippocampus, the part of the brain associated with memorized facts, and the neocortex, where the slow electrical oscillations begin. During deep sleep, bursts of activity in the hippocampus are synchronized with the slow wave oscillations originating in the neocortex.

So Born and his team devised an experiment to test whether the electrical oscillations were affecting memory consolidation. They asked a group of medical students to memorize a list of words. They then attached electrodes to the students' scalps and allowed them to go to sleep. As the students entered phases of deep sleep, the researchers pulsed the students' neocortexes with electrical currents at the same frequency as would normally been seen during that part of the sleep cycle. “Essentially we wanted to mimic the phenomenon,” Born says.

The team stimulated the brain for 5 minutes, stopped for 1 minute to record brain activity, and then stimulated the brain for another 5 minutes, during a 30-minute session, about the length of one slow-wave sleep cycle. “We used the one-minute intervals so that we could see the immediate effect of our stimulation,” explains postdoc Lisa Marshall, who conducted the experiment.

The researchers found that the neocortex produced more pronounced slow wave oscillations after stimulation. “We were surprised about the size of the effect,” says Born. After the students woke up, those who had been 'stimulated' could remember more words than the those who had not had the treatment.

Born speculates that the slow wave oscillations are transmitted to neurons in the hippocampus, causing the cells to synchronize their firing. The resulting rhythmic activity may then provide a mechanism to relay 'quanta' of information back to the neocortex during memory consolidation. The stronger the oscillations, the better the communication between these two parts of the brain, and the more memory consolidation.

As tempting as it may sound, Born warns that students should not try the experiment at home. “We did this study to show that the slow wave oscillations are relevant to memory. Medical students have lots of slow wave oscillations of their own, which are sufficient to remember things.”