Getting a good night’s sleep has long been known to cement the day’s memories, moving them from short-term storage into long-term holding, but new research shows that it’s not automatic. A night of z’s is helpful only if you know a test is coming or, more generally, if you explicitly tell yourself you’ll need the information in the future. In other words, don’t expect eight hours of shut-eye to help you on a pop quiz.
In an elegant series of experiments, scientists at the University of Lübeck in Germany tested memory by having volunteers learn 40 word pairs, or the location of 15 cards in a Concentration-type game plus a sequence of finger taps (pinkie, index, forefinger…). Sleep improved retention only in those who had been told they’d be tested 10 hours later, not in those for whom the quiz came as a surprise, says a report in the Journal of Neuroscience. “Merely expecting that a memory will be used in a test determines whether sleep benefits its consolidation,” says Lübeck’s Jan Born.
The new research is the first to show how sleep works its memory magic. EEGs found that the “test is coming” group spent more time in deep, slow-wave sleep than did the group not anticipating a test. Slow electrical waves act as a replay button, causing the hippocampus to reactivate new memories and synchronizing the neocortex so that it accepts them into long-term storage. This expectant group also had more “sleep spindles,” bursts of electrical activity that prime networks in the cortex to store memories arriving from the hippocampus and to integrate them into existing knowledge, which makes retrieval easier.