Objective: There is robust evidence for the influence of sleep disturbances on the maintenance of PTSD symptoms. However, little is known about day-to-day variation in trauma-related sleep disturbances (namely insomnia symptoms and nightmares) and their associations with PTSD symptoms. Therefore, we explored the dynamic interplay of these symptoms in daily life using an experience sampling method (ESM). Method: For 15 consecutive days, participants with PTSD symptomatology as primary complaint (N = 48) reported momentary levels of insomnia symptoms and nightmares as well as PTSD symptoms via a mobile app. Results: Multilevel model analyses revealed that insomnia and nightmares were significant predictors of PTSD symptomatology on the following day; furthermore, nightmares were predictive of each of the four PTSD symptom clusters, namely reexperiencing, avoidance, cognition and hyperarousal as well as symptoms of dissociation. However, PTSD symptoms did not predict insomnia or nightmares during the following night. Multilevel mediation analyses suggested that nightmares mediate the relationship between insomnia and next-day PTSD symptoms. Conclusions: These findings support accumulating evidence that trauma-related sleep disturbances play an important role in the maintenance of PTSD symptoms, by elevating symptoms daily.