Devices, systems and methods to modify memory and/or cognitive function by delivering a sensory stimulus paired with learned material at opportune physiological periods during sleep. For example, described herein are systems, methods and devices to enhance a user's cognitive function in such areas as memorization and learning. A machine (e.g., a system or device) may be used to identify opportune periods of the sleep cycle and to deliver a stimulus during specific phases of the sleep cycle to facilitate or interrupt memory consolidation. In some variations the machine records ambient sensory inputs during awake acquisition or reinforcement/relearning and replays all or an extracted form of the ambient sensory stimuli a specified portion of the user's sleep.