PTSD is a growing concern for both active duty personnel and Veterans. Fear conditioning is implicated in the development of PTSD, while successful acquisition, consolidation, and recall of extinction memory are implicated in both the natural reduction of initial PTSD symptoms and as the mechanism underlying the most successful treatment for PTSD, Prolonged Exposure. This project is the first to examine the role of sleep and sleep loss in acquisition, consolidation, and generalization of extinction memory in humans. Our main finding is that sleep loss most strongly affects recall of extinction learning, and that the REM sleep stage is associated with ability to recall extinction as well as recall safety signal learning. These findings suggest that sleep plays a critical role in long term retention of fear inhibition processes, and efforts to support sleep and especially adequate REM during exposure therapy may enhance efficacy and reduce remission after treatment.