Almost a century after the discovery that sleep helps us remember things, scientists are beginning to understand why.

During sleep, the brain produces chemicals that are important to memory and relives events we want to remember, scientists reported this week at the Society for Neuroscience meeting in Washington D.C.

"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. Yet this connection hasn't been well understood, he says.

That's changing, though, thanks to recent research from scientists including Jennifer Choi Tudor from the University of Pennsylvania. At the meeting, Tudor presented a study involving a brain chemical (known as 4EBP2) that is produced during sleep and is thought to play a role in remembering new information.

Previous experiments have shown that sleep-deprived mice have memory problems and lower levels of this chemical. So the team tried injecting the chemical into the brains of mice, then deprived them of sleep. "With the injection, their memory is normal," Tudor says.

To sleep, perchance to learn

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 the rats were awake, as well as during sleep.

When the animals were awake and traveling around their cages, the scientists identified brain cells that became active only when the rats were in a specific location. During sleep, these same cells became active in the same order, indicating 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 center in the animal's brain each time the brain cell associated with a specific location became active. The idea was to form, in the brain, a positive association with one place in the cage. And sure enough, when the animals woke up they went straight to that location, looking for a pleasurable reward.

The finding not only shows that new memories can be formed during sleep, Benchenane says, it suggests a new way to treat people who have post-traumatic stress disorder, and often have a negative association with a particular experience. It might be possible to eliminate that negative association during sleep by providing a pleasurable feeling every time they dream about that experience.

The food-sleep-memory connection

One surprising bit of research at the meeting was a study that suggests a midnight snack can undo the memory benefits usually conferred by getting enough sleep. A team from UCLA found that mice that ate during their normal sleep time scored worse on memory tests than mice that ate during their normal waking hours.

"Those animals show severe deficits in their recall," says Christopher Colwell. And the deficit occurred even if the animals were getting a normal amount of sleep, he says. The finding suggests that people who wake up during the night and want to snack should probably abstain, Colwell says, if they want their memory to work normally the next day.