The consolidation of memory traces during sleep for their long-term storage in the cortex has not been demonstrated directly. Maingret et al. trained rats either in a spatial memory task that required consolidation or a task that did not. They found that hippocampo-cortical oscillatory coupling during sleep increased only following training in the task that required memory consolidation. Moreover, artificially boosting this temporal coordination during sleep increased the responsiveness of prefrontal neurons to the task and improved next-day performance in the task, suggesting that hippocampo-cortical communication during sleep is important for memory consolidation.