A popular theory suggests that sleep helps "fix" memories in the brain, and new research finds that passing a gentle electric current through the sleeping brain improves memory even more.

In this new study, researchers show that using an electric current at a particular frequency during non-rapid-eye-movement (non-REM) sleep can enhance memory about 8 percent.

"Students we tested were better at remembering a vocabulary list with oscillating electric stimulation," said lead researcher Jan Born, from the Department of Neuroendocrinology at the University of Luebeck, in Germany. "Between stimulation we observed increased slow oscillating activity, which is generated by the brain itself. This intensifies slow-wave sleep," he said.

His team reported the findings in the Nov. 5 online edition of Nature.

Intensifying slow-wave sleep enhances memory consolidation, Born said. Electric stimulation intensifies the brain's own slow oscillating activity, which drives the replay of recent memories. "This replay is the way memory is enhanced and retained," Born said.

In the study, Born's team applied electric stimulation to the scalps of 13 medical students who had been given a list of words to memorize before sleeping. While the students slept, five jolts of the current -- oscillating at frequencies similar to those seen naturally in slow-wave sleep -- were given over half an hour.

The researchers found that, at that frequency, students had an 8 percent better memory of the words on the list. However, if the frequency of the current or the phase of sleep in which it was given was changed, there was no improvement in memory.

It is possible that electric stimulation might enhance the memory of older patients or patients with Alzheimer's, Born said. "We can improve the function of slow-wave sleep and therefore memory," Born said. "It could also have an application in sleep disturbances, because this stimulation also improves sleep."

One expert thinks that these findings add to the evidence that sleep is involved in memory and learning.

"This is an interesting article that buttresses the important argument that sleep is an active, not a passive, state," said Dr. Robert D. Vorona, an associate professor in the Division of Sleep Medicine at Eastern Virginia Medical School. "We are beginning to understand that specific stages of sleep and regions of the brain likely play a role in different types of learning."

In addition, it raises the question as to whether one might eventually be able to alter the electrical and chemical aspects of sleep to optimize learning, Vorona said. "For now, however, both adequate amounts of study and sleep are important for learning," he said.

Another expert isn't sure that memory and sleep are connected.

"I am skeptical about sleep-learning studies in general," said Jerome M. Siegel, a professor of psychiatry and member of the Brain Research Institute at the University of California Los Angeles Medical Center. "This is a popular idea now, but it doesn't fit with a lot of other data."

Siegel noted that sleep-deprivation studies have shown that lack of sleep has no effect on memory of things such as word lists, names or events. "In sleep, brain metabolism is very low," Siegel said. "The idea that the brain is constructing memories at a time of reduced metabolism is not very compelling."

In cross-species studies, Siegel has also found there is no correlation between learning and the amount of sleep. "The animal that sleeps the most is the bat; the animal that sleeps the least is the elephant; I don't think that people think that bats are more intelligent than elephants," he said.

And no correlation between sleep and intelligence has been found in human studies, either, Siegel added.

"If you look at human sleep, in relation to other animals, both total sleep and REM sleep, the percent of sleep devoted to REM sleep is not unusual," he said. "If you are egotistical enough to believe that humans are the smartest species, it sure doesn't show in our sleep."

Sleep may be involved in memory consolidation, Siegel said, but it is not essential to strong memories.