Dreaming when we sleep helps us learn new information, and now a study has found that it’s the boring dreams we have during the deepest stages of sleep that are the most important for this.
When we go to sleep, our brain-waves slow down and we enter progressively deeper stages of sleep, before returning to lighter sleep and entering a rapid-eye-movement (REM) stage. This cycle is repeated several times a night.
Until recently, we thought dreams only occurred during REM sleep. We now know that’s not the case, although REM dreams are the story-like, vivid ones we tend to remember. Those we have during deeper, non-REM sleep appear to be simpler and vaguer. When you remember being chased down the street by a dinosaur, that’s from a REM dream, says Björn Rasch of the University of Fribourg, Switzerland.
To investigate the role of sleep in replaying memories and helping people learn, Rasch and his colleagues recruited 22 volunteers. They were all asked to learn a list of one hundred words that were each linked to a picture, for example, the word tree with a picture of a child sitting on a chair.
That night, the team used electrode caps to track what stage of sleep each person was in. Throughout the night, the team regularly woke participants up and asked them what they had been dreaming about.
The following day, the volunteers were tested on the word-picture pairs. The team found that people did best on the memory task when they saw more of the pictures in their non-REM sleep. There was no such link when it came to REM sleep.
Another night, they were given a new memory task and then allowed to sleep undisturbed. They told the team about the dreams they could remember the following morning – and none of these remembered dreams contained pictures from the task.
These results are the first hint that dreaming about things during non-REM sleep might be more important than in REM sleep, says Rasch.