Evidence from animal and human research shows that established memories can undergo changes after reactivation through a process called reconsolidation. Alterations of the level of the stress hormone cortisol may provide a way to manipulate reconsolidation in humans. Here, in a double-blind, within-subject design, we reactivated a 3-day-old memory at 3:55 a.m. in sixteen men and four women, immediately followed by oral administration of metyrapone vs. placebo, to examine whether metyrapone-induced suppression of the morning cortisol rise may influence reconsolidation processes during and after early morning sleep. Crucially, reactivation followed by cortisol suppression vs. placebo resulted in enhanced memory for the reactivated episode tested four days after reactivation. This enhancement after cortisol suppression was specific for the reactivated episode vs. a non-reactivated episode. These findings suggest that when reactivation of memories is immediately followed by suppression of cortisol levels durin...