To sleep, perchance to remember: Many researchers believe sleep helps us consolidate our memories, but no one knows quite how. Yang et al. investigated the precise role of sleep in changing mouse brain structures (see the Perspective by Euston and Steenland). When mice learned motor tasks, small protuberances—or “spines”—formed on some of the dendritic branches of specific brain neurons. These spines represent the physical correlate of a memory. But the neurons grew and retained these spines better when the mice slept after learning the task. Neurons that fired during learning fired again during subsequent slow-wave sleep, allowing the mice to conserve the newly formed spines—and memories. Science , this issue p. 1173 ; see also p. 1087