A 45-minute midday nap can help boost your memory and remember facts, but only if you learned them well in the first place, a new study suggests.

This type of memory is called "declarative memory" and applies to standard textbook learning and knowledge, in contrast to "procedural memory," which applies to skills. Sleep appears to help "set" these declarative memories and make them easier to recall, the researchers said.

"Sleep appears to have an impact on what is learned well, but not so much when one is not motivated to learn," said lead researcher Matthew A. Tucker, a postdoctoral fellow at Harvard Medical School's Center for Sleep and Cognition.

For the study, 33 people were trained with certain declarative memory tasks. After the training, 16 took a non-REM nap, while 17 stayed awake and watched a movie. Later the same day, all the participants were tested. The tests included memorizing words, memorizing a maze and memorizing a complex line drawing.

Tucker's team found that over three very different declarative memory tasks, taking a nap improved performance compared with staying awake. However, napping only worked for people who had really learned the task well in the first place. - See A Daytime Nap Can Boost Memory for the complete article.