How sleep helps us learn

A Most research into the relationship between memory and sleep has traditionally been conducted using young adults or animals. By the early 2000s, scientists had found that sleep helps young adults consolidate memory by reinforcing and filing away daytime experiences. But the older adults that Rebecca Spencer was studying at the U.S. University of Massachusetts Amherst didn't seem to experience the same benefit. Spencer wondered if age altered the relationship between sleep and memory, and chose nearby preschool children as subjects. She found that the children who regularly had short sleeps during the day benefited the most from daytime rest, largely because their memories decayed the most without these naps. "By staying awake, they have more interference from daytime experiences," Spencer explains.

B TThe studies on young adults carried out in the early 2000s suggested that the reduced sensory inputs during sleep allow the brain to replay daytime experiences during a period relatively free of distracting information. This may help to solidify connections and transfer daytime memories from one part of the brain known as the hippocampus into long-term storage in the brain region called the cortex. But how sleep and memory interact at different periods of our lives remained an open question.

C In children younger than 18 months, learning is thought to occur in the cortex because the hippocampus isn't yet fully developed. As a result, researchers hypothesize that infants don't replay memories during sleep, the way adults do. Instead, sleep merely seems to prevent infants from forgetting as much as they would if they were awake. "The net effect is that sleep permits infants to retain more of the redundant details of a learning experience," says experimental psychologist Rebecca Gómez of the University of Arizona. "By the time they are two years old, we think that children have the brain development that supports an active process of consolidation," she adds.

D From the age of two, adequate sleep during the hours of darkness becomes critical for learning. Toddlers who sleep less than 10 hours display lasting cognitive deficits, even if they catch up on sleep later in their development. The effects are particularly strong in children with developmental disorders, who often suffer from disturbed sleep. Jamie Edgin of the University of Arizona studied children with the genetic disorder Down's syndrome, comparing those who were sleep-impaired with those who slept normally. She found that there were large differences in language knowledge and observed that the non-sleep-impaired children knew up to 190 more words, even after controlling for behavioural differences.

E Understanding the impact of sleep on memory could also help another at-risk group of learners at the other end of the age spectrum. Previous research has suggested that older adults don't remember recently acquired motor skills as well as young adults do. But neuroscientist Maria Korman and her colleagues at the University of Haifa in Israel recently demonstrated that taking a nap soon after learning can allow the elderly to retain procedural memories just as well as younger people. Korman hypothesizes that by shortening the interval between learning and consolidation, the nap prevents intervening experiences from weakening the memory before it solidifies. Overnight sleep might be even better, if the motor skills—in this case, a complex sequence of finger and thumb movements on the non-dominant hand—are taught late enough in the day.

F Optimizing the timing of sleep and training in the elderly exploits something Korman sees as a positive side of growing old. "As we age, our neural system becomes more aware of the relevance of the task," Korman says. Unlike young adults, who solidify all the information they acquire throughout the day, older people consolidate those experiences that were tagged by the brain as very important.

G Tests on older adults' memories are generating new findings about the relationship between sleep and memory at other ages as well. After learning at a conference about a memory test for cognitive impairment and dementia in older adults, neuroscientist Jeanne Duffy of Brigham and Women's Hospital in Boston wondered if sleep could help strengthen the connection between names and faces. She and her colleagues found that young adults who slept overnight after learning a list of 20 names and faces showed a 12 percent increase in retention when tested 12 hours later, compared with subjects who didn't sleep between training and testing. The findings have "an immediate real-world application," Duffy says, as they address a common memory concern among people of all ages.

H Developing a fuller picture of what happens to memories during sleep—and how best to modify sleep habits to aid the recall process—could benefit some of society's most sleep-deprived members of every age. "We need to understand this role of sleep in memory because there is such potential for intervention," Spencer says. "Now that we have a well-founded concept of what sleep can do for memory, it's time to put it to the test."