When you slip into sleep, it’s easy to imagine that your brain shuts down, but University of Michigan research suggests that groups of neurons activated during prior learning keep humming, tattooing memories into your brain. U-M researchers have been studying how memories associated with a specific sensory event are formed and stored in mice. In a study conducted prior to the coronavirus pandemic and recently published in Nature Communications, the researchers examined how a fearful memory formed in relation to a specific visual stimulus. They found that not only did the neurons activated by the visual stimulus keep more active during subsequent sleep, sleep is vital to their ability to connect the fear memory to the sensory event. Previous research has shown that regions of the brain that are highly active during intensive learning tend to show more activity during subsequent sleep. But what was unclear was whether this “reactivation” of memories during sleep needs to occur in order to fully store the memory of newly learned material.