Study Shows How Sleep Improves Memory

A good night's sleep triggers changes in the brain that help to improve memory, according to a new study led by researchers at Beth Israel Deaconess Medical Center (BIDMC).

These findings, reported in the June 30, 2005, issue of the journal Neuroscience and currently published on-line, might help to explain why children – infants, in particular – require much more sleep than adults, and also suggest a role for sleep in the rehabilitation of stroke patients and other individuals who have suffered brain injuries.

"In this new research, by using functional magnetic resonance imaging (fMRI), we can actually see which parts of the brain are active and which are inactive while subjects are being tested, enabling us to better understand the role of sleep to memory and learning."

New memories are formed within the brain when a person engages with information to be learned (for example, memorizing a list of words or mastering a piano concerto). However, these memories are initially quite vulnerable; in order to "stick" they must be solidified and improved. This process of "memory consolidation" occurs when connections between brain cells as well as between different brain regions are strengthened, and for many years was believed to develop merely as a passage of time. More recently, however, it has been demonstrated that time spent asleep also plays a key role in preserving memory.

In this new study, twelve healthy, college-aged individuals were taught a sequence of skilled finger movements, similar to playing a piano scale. After a 12- hour period of either wake or sleep, respectively, the subjects were tested on their ability to recall these finger movements while an MRI measured the activity of their brain.

According to Walker, who is also an Assistant Professor of Psychiatry at Harvard Medical School, the MRI results showed that while some areas of the brain were distinctly more active after a period of sleep, other areas were noticeably less active. But together, the changes brought about by sleep resulted in improvements in the subjects' motor skill performance.

"The cerebellum, which functions as one of the brain's motor centers controlling speed and accuracy, was clearly more active when the subjects had had a night of sleep," he explains. At the same time, the MRIs showed reduced activity in the brain's limbic system, the region that controls for emotions, such as stress and anxiety.

"The MRI scans are showing us that brain regions shift dramatically during sleep," says Walker. "When you're asleep, it seems as though you are shifting memory to more efficient storage regions within the brain. Consequently, when you awaken, memory tasks can be performed both more quickly and accurately and with less stress and anxiety."

This new research may explain why children and teenagers need more sleep than adults and, in particular, why infants sleep almost round the clock.

"Sleep appears to play a key role in human development," says Walker. "At 12 months of age, infants are in an almost constant state of motor skill learning, coordinating their limbs and digits in a variety of routines. They have an immense amount of new material to consolidate and, consequently, this intensive period of learning may demand a great deal of sleep."

The new findings may also prove to be important to patients who have suffered brain injuries, for example, stroke patients, who have to re-learn language, limb control, etc.

"Perhaps sleep will prove to be another critical factor in a stroke patient's rehabilitation," he notes, adding that in the future he and his colleagues plan to examine sleep disorders and memory disorders to determine if there is a reciprocal relationship between the two.

Beth Israel Deaconess Medical Center