THURSDAY, Jan. 22, 2004 (HealthDayNews) -- To sleep, perchance to store memories.
New research in rats suggests slumbering animals devote brain power to filing away recollections of the previous day's events. The process could explain how some short-term memories turn into long-term ones while others vanish.
"The general message is that sleep is a very important thing," and not just because it helps us rest, says study author Sidarta Ribeiro, a neurobiology researcher at Duke University.
But one expert says the study relies on flimsy evidence and doesn't prove the controversial notion that our minds process memories when we aren't even conscious.
While sleep takes up about a third of our lives -- teenagers excepted -- researchers haven't figured out the purpose of so-called Rapid Eye Movement (R.E.M.) sleep, when dreams occur, or the deep, dreamless form of sleep known as slow-wave.
In the latter phase, those who are awakened "will not report a dream, but will report thoughts," Ribeiro says.
What's happening in the brains of humans and mammals who experience the same back-and-forth sleep patterns? Some researchers think they know.
"In the past 30 years, particularly in the past 10 years, a lot of evidence has accumulated that sleep is very important for learning," Ribeiro says. Once a memory is acquired, some experts say, sleep helps it become properly stored away in the brain.
Ribeiro and colleagues tested the sleep-memory theory by hooking up rats to brain scan machines. The rats, who were living in "boring cages," were then exposed to interesting new things -- Froot Loops (yes, the breakfast cereal), wooden posts with spikes on them ("they weren't supposed to like it, but they loved it," Ribeiro says), a small brush and a golf ball mounted on a spring.
The purpose was to intrigue the rats and then measure the reaction of their brains to the changes in their environments. "The animals got really excited and went on to explore the objects," Ribeiro says. But the objects were removed after an hour so researchers could study how the rats remembered the new experiences.
The findings appear in the Jan. 19 online issue of the Public Library of Science.
The researchers say they noticed that areas of the brain that were activated when the rats explored the objects also lit up when they slept, especially during slow-wave sleep. It's as if their brains were echoing their earlier experiences, but at a lower level of fidelity, Ribeiro explains. The brains of the rats may have been replaying the day's events and deciding what to remember, Ribeiro suggests.
Jerry Siegel, a sleep expert at the University of California at Los Angeles, isn't convinced. He says the analysis of the brain scan is flawed, and suggests the researchers may have "dredged out" a connection that doesn't exist. "I'm not implying that they're being dishonest, but it's easy to delude yourself into thinking you have something when you don't."
He added that the very idea that sleep contributes to learning is questionable because people deprived of R.E.M. sleep don't forget things. "This is the core issue: Do you really need sleep to learn things? Many people in the field are all over the ballpark in terms of what they claim."
Regardless of what's really happening in the brain during sleep, Ribeiro offers some advice that seems practical no matter what scientists believe. "If people need to learn while studying, the best way is to study and go to sleep, rather than study too much and stay awake," he says.
Indeed, another study released this week, in the Jan. 22 issue of Nature, linked sleep to learning. European researchers found patients were better able to figure out a mathematical problem if they had time to sleep on it.
More information
For more on sleep, visit the National Sleep Foundation or the American Academy of Sleep Medicine.
