Participants were asked to memorize the location of 50 images on a computer monitor. The images were shown one at a time, arranged on the screen. Each object was shown with a related sound — so subjects heard a meow with the picture of a cat, and a crashing noise coupled with the image of a gong. After studying the locations, the participants were asked to take a short nap in a recliner in an adjacent room.

The investigators monitored the sleepers' brain activity, and when the participants had entered deep sleep, researchers began playing a series of 25 of the sounds that the individual had heard earlier in the memory game. When the participants woke up about an hour later, they said they hadn't heard a thing. But the test results suggested otherwise. On average, each person did slightly better at remembering the correct locations of the 25 objects whose related sounds had been cued during sleep than those of the other objects. The sounds appeared to have entered the sleeping brain and helped consolidate associated memories.

The prevailing theory is that during deep sleep, the brain replays certain experiences from the day, which, in turn, strengthens the memory of what happened. It is thought that when it comes to factual memories, like names, faces, numbers or locations, memory consolidation happens only during deep sleep — a phase of non–rapid eye movement sleep. (The other broad type of sleep, called rapid eye movement or REM sleep, which is when dreaming occurs, is believed to play a role in consolidating memories involving emotions and motor skills, such as dancing or playing an instrument.)

Although researchers are still a long way from understanding exactly how sleep affects memory, they are certain that getting too little sleep is a detriment. In other recent studies, it was found that people who had skipped a night's sleep fared worse at making new memories the next day, compared with those who had gotten a good night's sleep. Turns out, Mom's advice may have been right all along.

TIME MAGAZINE