Memory representations are reactivated during slow-wave sleep (SWS) after learning, and these reactivations cause a beneficial effect of sleep for memory consolidation. Memory reactivations can also be externally triggered during sleep by associated cues which enhance the sleep-dependent memory consolidation process. Here, we compared in humans the influence of sleep periods (i) of 40min and (ii) of 90min without externally triggered reactivations and (iii) of externally triggered reactivations by an associated odor cue during a 40-min sleep period on the consolidation of previously learned hippocampus-dependent visuo-spatial memories. We show that external reactivation by an odor cue during the 40-min sleep period enhanced memory stability to the same extent as 90min of sleep without odor reactivation. In contrast, 40min of sleep without external reactivations were not sufficient to benefit memory. In the 90-min sleep condition, memory enhancements were associated with time spent in SWS and were independent of the presence or absence of REM sleep. These results suggest that the efficacy of hippocampus-dependent memory consolidation depends on the duration of sleep and particularly SWS. External reactivation cues can accelerate the consolidation process even during shorter sleep episodes.
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