Memories are consolidated and strengthened during sleep. Here we show that memories can also be weakened during sleep. We used a fear-conditioning paradigm in mice to condition footshock to an odor (conditioned stimulus (CS)). Twenty-four hours later, presentation of the CS odor during sleep resulted in an enhanced fear response when tested during subsequent wake. However, if the re-exposure of the CS odor during sleep was preceded by bilateral microinjections of a protein synthesis inhibitor into the basolateral amygdala, the subsequent fear response was attenuated. These findings demonstrate that specific fear memories can be selectively reactivated and either strengthened or attenuated during sleep, suggesting the potential for developing sleep therapies for emotional disorders.