Individuals with posttraumatic stress disorder (PTSD) often report sleep disturbances such as trouble falling or staying asleep and repeated nightmares about the trauma. Laboratory-based polysomnographic studies have examined objective sleep abnormalities in PTSD; however, findings of these studies have been inconsistent. Researchers have speculated that artificial lab environments may have affected sleep in PTSD patients, and consequently contributed to the inconsistent findings. Further, despite basal hyperarousal in the autonomic nervous system (ANS) found in numerous lab studies with PTSD patients, possible associations between ANS hyperarousal and sleep disturbances in individuals with PTSD have not been fully understood due to a small number of ambulatory monitoring studies examining ANS activity in PTSD and also due to a lack of studies examining relationships between these variables. The present study examined sleep disturbances, ANS activity, and associations between these variables in 42 trauma-exposed college students with and without current PTSD symptoms (symptomatic and non-symptomatic groups) by simultaneously monitoring sleep and ANS activity for approximately 24 hours using ambulatory monitoring devices (actigraphy and inter-beat-interval monitor). Results revealed no significant differences between the symptomatic and non-symptomatic groups in sleep parameters measured by actigraphy [time in bed, total sleep time, sleep onset latency, wake after sleep onset (WASO), and sleep efficiency] or ANS arousal levels during sleep. However, results suggested significant interaction effects between PTSD symptom status and WASO on parasympathetic nervous system (PNS) activity levels during sleep and changes in PNS activity over time. Specifically, we found blunted increases in PNS activity during sleep in symptomatic individuals with longer WASO compared to symptomatic individuals with shorter WASO. Such WASO-related variations in PNS activity levels were smaller in non-symptomatic participants. These findings may suggest that diminished PNS activity during sleep is not found in all individuals with PTSD symptoms, but only in a subgroup of symptomatic individuals with long WASO. Further research is needed to identify characteristics of the subgroup of trauma victims who are vulnerable to sleep disturbances accompanied by nocturnal hyperarousal.