The researchers from the Harvard Medical School found that sleeping could strengthen the memories we made during the day.

They chose the study participants and divided them into four groups. All participants were supposed to memorize word pairs. The first two groups had to remember just one group of word pairs. One of these two groups was tested before they went home for a night sleep while the other group was tested the following morning after having a night sleep in their homes. Their test scores were similar.

The other two groups had been shown another set of word pairs similar to those they had to remember just before the testing to see whether their memory was distracted by the new set of word pairs. They were tested on both sets of word pairs.

The students who slept at home before the tests correctly identified about three-quarters of the word pairs while the students who took the test before going home for the evening correctly identified only about one-third of the word pairs.
The researchers report that this was the first study that showed how sleep protects memories from interference. They now believe that sleep disorders may be worsening memory problems seen in dementia.