Let sleeping mice remember The role of REM (rapid eye movement) sleep for memory consolidation has been discussed for a long time. Boyce et al. used optogenetics to inhibit theta oscillations in the mouse hippocampus during REM sleep (see the Perspective by Kocsis). Both object recognition memory and contextual fear memory were impaired. This consolidation mechanism occurred in a critical time window immediately after training. Disrupting the same system for similar durations during non-REM sleep or wakefulness had no effect on memory. Science , this issue p. 812 ; see also p. 770