REM sleep is known to help solidify memories, but the mechanism for making memories more permanent is not well-understood. A recent study published in Nature Neuroscience shows that, during REM sleep, some of the structures neurons use to make connections with each other are pruned, while others are maintained and strengthened. The findings indicate that sleep's role in solidifying memories comes through allowing the brain time to selectively eliminate or maintain newly formed neural connections.

Dendritic spines are small outgrowths on a neuron's dendrite, which is the portion of the neuron that receives chemical signals from other neurons. These spines enhance the strength of connections between neurons so they can play an important role in strengthening new neural circuits and solidifying new memories. These spines aren't permanent structures; instead, nerve cells can create new ones or get rid of existing ones (a process called pruning) as the importance of different connections shifts.

The new memories in this case were formed in mice, which were trained to complete a treadmill-like motor task. Then, the mice were either deprived of REM sleep or allowed to experience this form of sleep. The mice that were allowed REM showed significantly higher pruning of new dendritic spines compared to the mice that were REM sleep deprived. This difference in pruning was only seen for new dendritic spines, and previously existing dendritic spines were pruned at the same rate.