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.
"Our previous studies demonstrated that a period of sleep could help people improve their performance of 'memory tasks,' such as playing piano scales," explains the study's lead author Matthew Walker, PhD, director of BIDMC's Sleep and Neuroimaging Laboratory. "But we didn't know exactly how or why this was happening.
"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."
The end result is that procedural skills – for example, learning to talk, to coordinate limbs, musicianship, sports, even using and interpreting sensory and perceptual information from the surrounding world -- become more automatic and require the use of fewer conscious brain regions to be accomplished.
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.
"If you look at modern society, there has in recent years been a considerable erosion of sleep time," says Walker. Describing this trend as "sleep bulimia" he explains that busy individuals often shortchange their sleep during the week – purging, if you will – only to try to catch up by "binging" on sleep on the weekends.
"This is especially troubling considering it is happening not just among adults, but also among teenagers and children," he adds. "Our research is demonstrating that sleep is critical for improving and consolidating procedural skills and that you can't short-change your brain of sleep and still learn effectively."
Study co-authors include BIDMC researchers Gottfried Schlaug, MD, PhD, Robert Stickgold, PhD, David Alsop, PhD and Nadine Gaab, PhD.
This study was supported by grants from the National Institutes of Health and the Dana Foundation.
Beth Israel Deaconess Medical Center is a patient care, teaching and research affiliate of Harvard Medical School, and ranks third in National Institutes of Health funding among independent hospitals nationwide. BIDMC is clinically affiliated with the Joslin Diabetes Center and is a research partner of Dana-Farber/Harvard Cancer Center. BIDMC is the official hospital of the Boston Red Sox. For more information, visit www.bidmc.harvard.edu.