Memory consolidation refers to processes following the encoding of new information and allowing the transformation of labile and interference-sensitive memory traces into stable representations. The role of sleep in these processes was highlighted more than one century ago, and if the modalities of this crucial function (passive, permissive or active role), the type of memory concerned, and the respective role of different sleep stages have been highly debated, the complementary involvement of different vigilance stages-associated specific cerebral activity in memory stabilization and re-organization is now widely accepted. Several arguments have been provided toward an active role of sleep in memory consolidation:

– deleterious effect of post-learning sleep deprivation (or of several sleep disorders) on delayed recall performance;

– post-learning sleep structural changes, depending on the learning task type and correlated with delayed recall performance;

– improvement in memory performance induced by neuromodulation or targeted reactivation techniques applied during sleep.

The hippocampus plays a key role in these processes via sharp wave-ripples, high frequency oscillations associated with reactivation of neuronal activity sequences, synchronized with Non-Rapid Eye Movement sleep (N-REM) oscillations such as spindles and slow waves. This temporally fine-tuned coupling allow transfer of information from hippocampus to neocortex. Recent works argue for a sequential involvement of NREM and REM sleep: irrelevant information are tagged to be erased (synaptic downscaling), while other synapses are reactivated and potentiated, allowing then new memories to be incorporated into old representations and providing condition for their long-term stabilization.