College students studying foreign languages should take heed of a study recently published by researchers at Harvard Medical School and the University of York, which found that newly acquired words are better integrated in a person’s memory after sleep.

University of Manchester research associate Jakke Tamminen—who spent three months at Harvard last year—and his team studied how brain activity during sleep affects integrating new memories, drawing on previous research about how sleep strengthens memory in general.

The researchers taught volunteers new, made-up words that sounded similar to existing words, such as “cathedruke.”

The volunteers were tested on the words, slept overnight in the lab while researchers measured their brain activity, and then were tested again in the morning. The second test showed that volunteers could recognize more words and at a greater speed after sleep.

Members of the control group, who were tested in the morning and the evening after completing a normal day, were not able to recognize the words more quickly than they had in the morning.

“When you’re just learning a new word, it’s not yet in your brain in a way that it is after sleep,” said Robert A. Stickgold ’66, a co-author of the study and an associate professor of psychiatry at Harvard Medical School.

The researchers found that people who experienced the greatest number of sleep spindles exhibited the most significant changes in integration. Sleep spindles are intense bursts of brain activity that reflect how information transfers from deep within the brain in the hippocampus, to the neocortex at the surface.

While the hippocampus stores new memories separately from old ones, the neocortex connects these memories.

Recognition of familiar words was slowed in both groups, which indicates how the new words had been integrated in the volunteers’ “mental dictionary,” Tamminen said. The spindle activity showed how integration worked overnight, but not how recognition speed slowed over time. Tamminen said he is considering other experiments to measure the degree to which new memories are integrated with existing ones.

The study expanded on several previous studies led by M. Gareth Gaskell, a professor in the University of York’s Department of Psychology. Gaskell was originally interested in how new words become a part of the recognition system.

“We had all these bits and pieces that suggested an important part of learning language was involved in sleep and transfer in systems, so we wanted to look at brain processes during sleep,” Gaskell said.

Tamminen is currently expanding upon the study by having both volunteer groups at the lab for 90 minutes to better monitor their activities.

Fei Xu, a professor in the Department of Psychology at the University of California, Berkeley, said she saw this as an innovative study and suggested further research on the topic.

“Testing school-aged children might be interesting to extend this study to children who are learning new words and see how sleep affects their memory,” Xu said.

The study was published in the Journal of Neuroscience.