The study, which involved adolescents, has particular relevance, I believe, during this back-to-school week.
First, a bit of background: Until relatively recently, most scientists believed the sole function of sleep was to give our minds and our bodies a restorative rest. During the past two decades or so, however, research has revealed another critical reason for our nightly slumber: It appears to help our brains consolidate memories and, thus, acquire new information and skills.
Scientists have also made the fascinating finding that the timing of our sleep affects our learning. There seems to be a short period — just a few hours — in which the memory traces of newly learned information can be easily disrupted. Research also suggests, however, that if we fall asleep during that period, those fragile traces organize themselves in ways that strengthen the memories — and we are more likely to recall what we’ve learned at a later date.
In other words, the old advice to “sleep on it” seems to hold true when it comes to learning.
Testing two types of memory
But how much time should we leave between learning a particular type of information or skill and sleeping?
That brings us to the recent study. A team of German researchers recruited 50 teenage girls (aged 16 and 17). They brought them into a lab and trained them to learn a finger-tapping task and a series of word pairs, two tests commonly used in studies involving memory and learning. The finger-tapping task involves procedural (“knowing how”) memory — the kind that’s needed to learn such skills as playing a sport or a musical instrument. The word-pair task involves declarative (“knowing what”) memory — the kind that’s needed to learn factual information. These two memory systems use different parts of the brain.
Half of the teenage girls learned their tasks at 9 p.m., right before they went to sleep. The other half were trained at 3 p.m., about 7.5 hours before they went to sleep. (All the sleeping was done in the lab so that the researchers could verify the length and quality of the girls’ sleep.)
The teenagers who underwent the finger-tapping training in the evening session showed the greatest improvement in those skills when re-tested after 24 hours and again after seven days. For the word-pair test, the results were reversed: The girls trained in the afternoon did the best when re-tested 24 hours later, although this evidence was “less robust,” according to the study’s authors, and thus should be treated with some caution.
No difference was found between the afternoon and the evening learners when their newly acquired word-pair skills were tested seven days later.
Obviously, the study had several important limitations. It involved a small number of participants, for example, and all of them were girls. The findings may not hold in a larger and more diverse population. Still, the study’s results are provocative and may have some practical implications.
“Even though it should be borne in mind that this is the first study showing these results,” write the German researchers, “the findings might contribute to the development of new effective teaching and learning strategies. Translating the results to the everyday life of adolescents, we propose that declarative memories, such as vocabulary words, should be studied in the afternoon and motor skills, like playing soccer or piano, should be trained in the late evening.”
“Most parents among us,” the researchers added wryly, “would have preferred the opposite results.”