Almost a century after the discovery that sleep helps us remember things, scientists are now beginning to understand why. The Society for Neuroscience has discovered that it comes down to the active chemicals in your brain, while you are asleep.
During sleep, the brain produces chemicals which relive events that we want to remember; making the memories important and therefore more permanent. "One of the most profound effects of a night of sleep is the improvement in our ability to remember things," says Ravi Allada, a sleep researcher at Northwestern University.
"Sleep is also a time when old memories can be modified and new memories can be formed." says Karim Benchenane from the National Center for Scientific Research in Paris. Benchenane was part of a team that studied the brains of rats while they were both awake and asleep.
When the animals were awake and moving around their cages, the scientists identified brain cells that only became active when the rats were in a specific location. During sleep, these same cells became active in the same order, showing that the rats were reliving their travels and presumably strengthening their memories of places they'd been.
Then Benchenane's team set out to change each rat's memory during sleep. They did this by stimulating the pleasure centre in the animal's brain each time the brain cell associated with a specific location became active. The idea was to form a positive association with one place in the cage. Sure enough, when the animals woke up, they went straight to that location, looking for a positive reward.
"The finding not only shows that new memories can be formed during sleep, but it also suggests a new way to treat people who have post-traumatic stress disorder and often have a negative association with a particular experience." says Benchenane.
One surprising bit of research in the same study found that having a midnight snack can undo the memory benefits of getting enough sleep. The researchers found that people who wake up in the night and want to snack should try and resist if they want their memory to work normally the next day.