In this paper, we investigated ambulatory sleep heart period variability in panic disorder participants with nocturnal panic (NP) compared to daytime panic attacks only. A time-derived measure of heart period variability (HPV) during sleep was significantly reduced in the NP group (n=32) relative to the daytime panic (n=17) and nonanxious (n=17) control groups. Consistent with previous work, NP participants also reported greater fear of relaxation and sleep than daytime panic and control groups. Based on a neurovisceral model of attention [Thayer, J. F., & Lane, R. D. (2000). A model of neurovisceral integration in emotion regulation and dysregulation. Journal of Affective Disorders, 61, 210-216] that predicts that decreased HPV is related to disregulated behavioral adaptation, we hypothesized that HPV measured during sleep would be most reduced in NP participants. These findings indicate that HPV is related to nocturnal panic disorder insofar as it is measured during sleep.