Most individuals diagnosed with borderline personality disorder (BPD) have been exposed to severe and traumatic stressors and thus frequently present with symptoms of a posttraumatic stress disorder (PTSD). Severe sleep disturbances often accompany these complex cases, but changes of sleep parameters during therapy and the impact of sleep on treatment response have barely been studied. Narrative Exposure Therapy (NET) is an evidence-based approach for the treatment of trauma-related psychological disorders. To investigate the effect of NET on sleep in patients with BPD and comorbid PTSD, we screened 45 inpatients and outpatients who met the inclusion criteria of both diagnoses according to DSM-IV and who had a minimum of 2 weeks' stable medication. Patients were allocated to NET (N = 13) or treatment as usual (TAU; N = 8) in blocks. Polysomnographies and psychological questionares were performed before, directly and 6 months after the last therapy session. The aim of this pilot study was to investigate the effectiveness of trauma therapy by NET on sleep quantity (total sleep time) and sleep continuity (sleep efficiency and awakenings) in patients with comorbid BPD and PTSD. Participants of the NET group compared with those who received TAU showed an increased reduction in sleep latency from baseline to the end of therapy and a reduction in arousals over time. Patients with longer pre-treatment total sleep time and pre-treatment REM sleep duration showed a better outcome of NET with respect to PTSD symptoms. NET seems not lead to a change in sleep for the worse during therapy and seems to improve sleep as good as treatment as usual. Furthermore, our results provide evidence of an influence of sleep structure at baseline on treatment success later on.