The mystery of sleep continues to elude us. We know it is good for something (ask those who have tried to do without), but to pinpoint what that something is has not been easy. Slowly, the pieces of the puzzle are coming together, and it seems one of the functions of sleep is in the evaluation and remembrance of events of the day that has passed. A study in the The Journal of Neuroscience suggests that the brain evaluates memories during sleep and preferentially retains the ones that are most relevant based on expectations of what is likely to occur in the future.
A press release describes the work carried out by scientists at 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. As expected, the 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 electroencephalogram (EEG) readings 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 later for memory recall.
“The more slow wave activity the sleeping participants had, the better their memory was during the recall test 10 hours later,” notes a researcher. Scientists have long thought that sleep is important in memory consolidation. The authors suggest that the brain’s prefrontal cortex ‘tags’ memories deemed relevant while awake, and the hippocampus consolidates these during sleep.