The brain evaluates information during sleep and retains the ones it knows could be useful in the future, suggests a new study.
The study has shed light on how the brain decides what to keep and what to forget.
"Our results show that memory consolidation during sleep indeed involves a basic selection process that determines which of the many pieces of the day's information is sent to long-term storage. Our findings also indicate that information relevant for future demands is selected foremost for storage," said lead author Jan Born, of the University of Lubeck in Germany.
The researchers set up two experiments to test memory retrieval in a total of 191 volunteers. In the first experiment, people were asked to learn 40 pairs of words. Participants in the second experiment played a card game where they matched pictures of animals and objects - similar to the game Concentration - and also practiced sequences of finger taps.
In both groups, half the volunteers were told immediately following the tasks that they would be tested in 10 hours. In fact, all participants were later tested on how well they recalled their tasks.
Some, but not all, of the volunteers were allowed to sleep between the time they learned the tasks and the tests.
They found that people who slept performed better than those who didn't. But more importantly, only the people who slept and knew a test was coming had substantially improved memory recall.
The researchers also recorded electroencephalograms (EEG) from the individuals who were allowed to sleep. They found an increase in brain activity during deep or "slow wave" sleep when the volunteers knew they would be tested for memory recall.
The findings were reported in the Journal of Neuroscience.