Sleep as a memory aid: It works best when you know you’ll need that information later

Remember the annoying classmate who would raise her hand regularly and ask the teacher, “Is this going to be on the final exam?” What if the answer were “no,” you wondered: was she going to take a nap for the rest of class?

Maybe she planned to take a nap after class if the answer were yes, ensuring that she’d store the class material optimally for the big test. Such a strategy would be a good bet, according to a new study appearing in the Journal of Neuroscience.

Sleep has long been known as an activity that aids in the retention of new information, which may be why late-night studying has always seemed like such a good idea to students. But which bits of information, in the feast of daily lessons we take in, deserve to be processed and stored during sleep for retrieval at some unknown future date? Which of the millions of bits of input deserves to become a long-term memory?

The bits we are told we will need again, as it turns out.

A group of German researchers set about exploring how sleep selects the memories it will package up and store for later retrieval. Some 193 study participants were put through their paces at learning tasks including finger-tapping sequences, card-pair locations and lists of matched words, and then either told they would be tested later on the material or not. Some subjects were exposed to the material early in the day, when there would be no sleep involved. Others were exposed to the material just before nighttime sleep.

Whether the prospective memory was “procedural” (a motor exercise such as finger-tapping) or “declarative” (having to do with explicit facts or associations), subjects who were told they would be tested on their recall of the material were more likely to remember the tasks accurately if they had slept after their exposure to the material than if they were awake afterward.

And among those who knew they would be tested on the materials, researchers routinely observed sleep patterns different from the sleep patterns of subjects who were not told of a future test: subjects prepared to be quizzed later on the tasks spent more of their sleep time in the deepest form of sleep, Stage 4, during which their brain waves oscillated in a distinctive, slow-wave motion. And the more pronounced these distinctive sleep patterns were, the better subjects remembered what they had been taught.

In this light, it may not have been rude of that classmate to ask whether something was going to be on the final. She just needed to know whether to put the material in the queue for storage during sleep that night.