U.S. researchers said Thursday they have found important physical evidence that sleep helps consolidate and strengthen new memories.

In a study published in the U.S. journal Science, researchers at New York University showed for the first time that sleep after learning encourages the growth of dendritic spines, the tiny protrusions from brain cells that connect to other brain cells and facilitate the passage of information across synapses, the junctions at which brain cells meet.

Moreover, the activity of brain cells during deep sleep called slow-wave sleep after learning is critical for such growth.

"We've known for a long time that sleep plays an important role in learning and memory. If you don't sleep well you won't learn well," senior investigator Wen-Biao Gan, professor of neuroscience and physiology of the university, said.

"But what's the underlying physical mechanism responsible for this phenomenon? Here we've shown how sleep helps neurons form very specific connections on dendritic branches that may facilitate long-term memory," Gan said.

Gan and colleagues employed mice genetically engineered to express a fluorescent protein in neurons.

Using a special laser-scanning microscope that illuminates the glowing fluorescent proteins in the motor cortex, they were then able to track and image the growth of dendritic spines along individual branches of dendrites before and after mice learned to balance on a spin rod.

They trained two sets of mice: one trained on the spinning rod for an hour and then slept for 7 hours; the second trained for the same period of time on the rod but stayed awake for 7 hours.