A little brain stimulation at night appears to help people remember what they learned the previous day.

A study of 18 people with severe epilepsy found that they scored higher on a memory test if they got deep brain stimulation while they slept, a team reports in the journal Nature Neuroscience.

The stimulation was delivered during non-REM sleep, when the brain is thought to strengthen memories it expects to use in the future. It was designed to synchronize the activity in two brain areas involved in memory consolidation: the hippocampus and the prefrontal cortex.

"Some improved by 10% or 20%, some improved by 80%," depending on the level of synchrony, says Dr. Itzhak Fried, an author of the study and a professor of neurosurgery at the University of California, Los Angeles.

The results back a leading theory of how the brain transforms a daily event into a memory that can last for days, weeks, or even years. They also suggest a new approach to helping people with a range of sleep and memory problems.

"We know for instance that in patients with dementia, with Alzheimer, sleep is not working very well at all," Fried says. "The question is whether by changing the architecture of sleep, you can help memory."

Although the results are from a small study of people with a specific disorder (epilepsy), they are "reason to celebrate," says Dr. György Buzsáki, a professor of neuroscience at New York University who was not involved in the research.

Rhythms in the brain

During sleep, brain cells fire in rhythmic patterns. Scientists believe that when two brain areas synchronize their firing patterns, they are able to communicate.

Studies suggest that during non-REM sleep, the hippocampus, found deep in the brain, synchronizes its activity with the prefrontal cortex, which lies just behind the forehead. That process appears to help transform memories from the day into memories that can last a lifetime.

So Fried and his team wanted to know whether increasing synchrony between the two brain areas could improve a person's memory of facts and events.

Their study involved epilepsy patients who already had electrodes in their brains as part of their medical evaluation. This gave the scientists a way to both monitor and alter a person's brain rhythms.

They measured memory using a "celebrity pet" test in which participants were shown a series of images that matched a particular celebrity with a specific animal. The goal was to remember which animal went with which celebrity.

Patients saw the images before going to bed. Then, while they slept, some of them got tiny pulses of electricity through the wires in their brains.

"We were measuring the activity in one area deep in the brain [the hippocampus], and then, based on this, we were stimulating in a different area [the prefrontal cortex]," Fried says.

In patients who got the stimulation, rhythms in the two brain areas became more synchronized. And when those patients woke up they did better on the celebrity pet test.

The results back decades of research on animals showing the importance of rhythm and synchrony in forming long-term memories.

"If you would like to talk to the brain, you have to talk to it in its own language," Buzsáki says.

But altering rhythms in the brain of a healthy person might not improve their memory, he says, because those communication channels are already optimized.

The epilepsy patients may have improved because they started out with sleep and memory problems caused by both the disorder and the drugs used to treat it.

"Maybe what happened here is just making worse memories better," Buzsáki says.

Even so, he says, the approach has the potential to help millions of people with impaired memory. And brain rhythms probably play an important role in many other problems.

"They are not specific to memory. They are doing a lot of other things," Buzsáki says, like regulating mood and emotion.

So tweaking brain rhythms might also help with disorders like depression, he says.