Prior research has demonstrated that individuals exposed to trauma have shown impaired autonomic function. We sought to determine if heart rate variability (HRV), a marker of impaired autonomic function, differed across periods of wake, rest, and sleep as a function of the level of symptoms of posttraumatic stress disorder (PTSD). A sample of young adults (N = 209), 95 of whom met full criteria for current PTSD based on the Clinician Administered PTSD Scale (CAPS; Blake et al., 1995), were evaluated for ≈ 24 hr using actigraphy and electrocardiogram. Actigraphy data were categorized as active, rest, or sleep. Multilevel modeling analyses showed that individuals with high PTSD symptom severity had lower high-frequency HRV than individuals with low PTSD symptom severity during periods of sleep, t(1083) = 2.20, p = .028, Cohen's d = 0.12. No differences were found during periods of activity, t(1083) = 1.34, p = .499, d = 0.05, or rest, t(1083) = 1.34, p = .180, d = 0.09. Our findings extended the import of prior studies to suggest that those with elevated PTSD symptoms may have decreased parasympathetic control during sleep. Moreover, relative to periods of wake and rest, sleep may represent a state of increased vulnerability for decreased parasympathetic cardiac control. Individuals with elevated PTSD symptoms may benefit from early screening for detection of cardiovascular disease.