Objective: Disturbed sleep is common among individuals with posttraumatic stress disorder (PTSD), but there has been limited research on the momentary relationships between daytime PTSD symptoms and nighttime sleep. The goal of this study was to examine the relationships between daytime peak PTSD symptoms and sleep duration that night and between sleep duration and peak PTSD symptoms the next day. Method: The study sample was 42 American post-2001 veterans recruited for a study of risky sexual behavior who completed a baseline PTSD Checklist-5 For 28 days, PTSD symptoms were assessed 3 times per day using a version of the PTSD Checklist-5 modified to ask about the previous 2 hours. Each morning, participants rated the previous night's sleep duration. Two multilevel models were estimated, 1 modeling a given day's peak PTSD symptoms and the other modeling a given night's sleep duration. Results: In the first model, peak PTSD symptoms on a given day were significantly related to mean peak daily PTSD symptoms, estimate = 1.003, p < .001; previous night's sleep duration, estimate = -1.799, p < .001; and previous day's peak PTSD symptoms, estimate = .159,p < .05. In the second model, sleep duration on a given night was associated with mean sleep duration, estimate = 1.032, p < .001, but not with peak PTSD symptoms during that day, estimate = -.001, ns. Conclusions: This study adds to research indicating that a poorer-than-usual night's sleep is associated with higher peak PTSD symptoms the next day but higher peak PTSD symptoms in the day are not associated with worse sleep that night.