A new study provides more structural evidence that sleep plays a role in memory and learning.
Researchers from New York University Langone Medical Center found that mice who sleep after learning a task have increased growth of dendritic spines, which come out of brain cells to connect with other brain cells and play a role in the passage of information across the brain’s synapses.
"Now we know that when we learn something new, a neuron will grow new connections on a specific branch," study researcher Wen-Biao Gan, Ph.D., a professor of neuroscience and physiology and a member of the Skirball Institute of Biomolecular Medicine at NYU, said in a statement. "Imagine a tree that grows leaves (spines) on one branch but not another branch. When we learn something new, it's like we're sprouting leaves on a specific branch."
The study, published in the journal Science, involved mice that were genetically engineered so that a protein in their neurons would glow. The researchers used a special microscope that illuminated these proteins in the mice’s motor cortex, in order to view the growth of the dendritic spines.
The researchers had the mice learn to balance on a spinning rod that increasingly spun faster, a learning situation similar to riding a bike in that once you learn it, you don’t forget it. After learning this task, the mice did indeed experience growth in dendritic spines.
Then, researchers wanted to see how sleep might affect the growth of these dendritic spines. They had one group of mice learn how to balance on the spinning rod and then sleep for seven hours, and then they had another group of mice take the same amount of time to balance on the spinning rod but then stay awake for seven hours.
Mice that were allowed to sleep after learning the spinning rod balancing task experienced more dendritic spine growth than the sleep-deprived mice, researchers found. Plus, depending on the type of task the mice learned -— such as running forward on the rod, or running backward on the rod —- dendritic spines grew on different dendritic branches.
“These findings indicate that sleep has a key role in promoting learning-dependent synapse formation and maintenance on selected dendritic branches, which contribute to memory storage,” the researchers wrote in the study.
Other research has also shown glimpses at how sleep affects learning and memory. Another study, also conducted by researchers at NYU, showed that rats’ memory of a particular smell is stronger when exposed to the smell during slow-wave sleep, versus when exposed to the smell when awake.
And a small study in people, conducted by Michigan State University researchers, suggested that sleep seems to be associated with improved working memory capacity, which could play a role in problem-solving, decision-making, and learning.