Getting better at something involves emotion. When we do well, we have good feelings—pride, pleasure, excitement—and these emotions help reinforce whatever behaviors we just engaged in. Similarly, the pain of failure makes recent behaviors less likely in the future. This is conditioning, and we’ve all experienced it—when we’re awake. But what about when we sleep? Sleep reinforces memories. We know this because after half an hour of sleep, people can remember things better than when they spend half an hour doing something else, like watching TV. Studies of rats show that their brains rehearse running through mazes while they sleep, in a process known as sleep replay. Memory’s function is to store information that will be useful. Because of this, our mind prioritizes remembering some things over others. Studies have shown, for example, that it’s easier to remember things that are useful for survival. Might sleep similarly focus on things that are particularly good or bad for us, like food and dangerous animals, and ignore things that are irrelevant to our well-being, like the exact shape of a cloud?
Our minds are rehearsing things during sleep and they preferentially feature things that are good for us over experiences that don’t matter to us. Does this have anything to do with dreaming? Well, none of the participants entered rapid-eye-movement (REM) sleep, which is when most dreams happen, during their time in the sleep lab. The neural replay happened only during slow-wave sleep when dreams tend to be infrequent, dull, and go unremembered. The participants might not even have been aware of the rehearsal their brains were engaged in. These rehearsals might not even be conscious!
Other evidence suggests that dreaming during REM sleep tends to be more negative—bad dreams are more common than good ones. This leads to the interesting idea that positive experiences are rehearsed during non-REM sleep, and negative ones are rehearsed during REM sleep.
Jim Davis
