A. In animals, memory reactivation is seen as patterns of cell firings during waking exploration of an environment (Top) that are reiterated in subsequent NREM sleep (Bottom), albeit on a faster time scale. Vertical bars represent the time of peak firing for 10 individual cells demonstrating clear place fields in the training environment [79]. B. In humans, evidence of reactivation has been reported in imaging studies demonstrating that brain regions engaged during task encoding are again active during post-training sleep. In this example, “reactivation” of hippocampal activity during post-training sleep (Right) predicted overnight improvement in memory performance on a spatial learning task (Left). Adapted with permission from Lee AK & Wilson MA. Memory of Sequential Experience in the Hippocampus during Slow Wave Sleep. Neuron 2002; 1183–94 & Peigneux P, Laureys S, Fuchs S, et al. Are spatial memories strengthened in the human hippocampus during slow wave sleep? Neuron 2004; 535–45.