(A) A rat traverses a track over about 20 s and encodes locations using hippocampal place cells (14 colour-coded units are shown). (B) The rat sleeps after running on the maze and a colour-coded Rasta plot shows the same place cells firing again in roughly the same order in which they were active during track running. Interestingly, the entire 20-s experience is replayed in about 200 ms during non-REM sleep. (C) A hypnogram showing a typical night of sleep, with sleep stages indicated on the y axis as: wake (W), REM with 4–9 Hz theta activity in the hippocampus, Stage 2 (N2) with a characteristic 14 Hz sleep spindle, and Stage 3 (N3) SWS. Time is on the x axis. Black ovals illustrate the morphology of slow oscillations, theta activity, and sleep spindles (from left to right). Abbreviations: REM, rapid eye movement; SWS, slow wave sleep.