Students are told to “sleep on it,” because many experiments have shown an early stage of sleep helps consolidate the memory of recently learned facts. Now German neuroscientists have found that stimulating the sleeping brain with external electric fields can further boost memory performance.

Jan Born and his colleagues at the University of Luebeck gave a simple memory test—sets of word pairs—to 13 volunteer medical students and then let them fall asleep. As expected, their ability to remember the words improved after a nap. “Slow-wave sleep,” Born says, “has been suspected to be the phase of sleep when memory consolidation occurs.” During this phase, the brain generates waves of neuronal activity that oscillate from front to back, about once per second. But whereas memories of facts are known to solidify during this slow-wave period, the electrical oscillations themselves were not thought to be important to the process.

Born's team applied electrodes to their subjects' heads and allowed them to doze off. When the students were about to enter slow-wave sleep, the scientists induced a slowly oscillating current that was slightly stronger than the brain's natural one. Five sessions of this stimulation spaced out over half an hour enhanced the slow waves, keeping them strong and synchronizing the activity of neurons. When the subjects awoke and were tested, they could remember almost three more word pairs than control subjects, who were given sham treatment.

Specific brain-wave patterns could also play a role in other sleep stages. Memories of skills, rather than facts, are consolidated during the later stage of rapid eye movement, or REM, sleep. The brain's electrical activity during this sleep stage is dominated by higher-frequency theta waves. Born's group now plans to see if artificially reinforcing theta waves during sleep will specifically boost this different kind of memory.