Quasi-causal associations between chronotype and post-traumatic stress disorder symptoms: A twin study.

Objective: The evening ("night owl") chronotype is associated with greater severity and lifetime prevalence of post-traumatic stress disorder (PTSD) symptoms compared to morning or intermediate chronotypes. This twin study investigated the gene-environment relationships between chronotype, recent PTSD symptoms, and lifetime intrusive symptoms.

Methods: We used the reduced Horne-Östberg Morningness-Eveningness Questionnaire (rMEQ) to assess chronotype in a sample of 3777 same-sex adult twin pairs raised together (70.4% monozygotic, 29.6% dizygotic) in the community-based Washington State Twin Registry. PTSD symptoms were reported on the Impact of Events Scale (IES) and a single item for lifetime experience of intrusive symptoms after a stressful or traumatic event.

Results: Genetic influences accounted for 50% of chronotype variance, 30% of IES score variance, and 14% of lifetime intrusive symptom variance. Bivariate twin models showed a phenotypic association (b) between evening chronotype and more severe PTSD symptoms (b = -0.16, SE = 0.02, p < .001) that remained significant even after adjusting for shared genetic and environmental influences (b = -0.10, SE = 0.04, p = .009), as well as age, sex, and self-reported sleep duration (b = -0.11, SE = 0.04, p = .004). An association was found between evening chronotype and lifetime intrusive symptoms (b = -0.11, SE = 0.03, p < .001) that was no longer significant after adjusting for shared genetic and environmental influences (b = 0.04, SE = 0.06, p = .558).

Conclusions: Our results suggest a "quasi-causal" relationship between evening chronotype and PTSD symptoms that is not purely attributable to genetic or shared environmental factors. Evening chronotype may increase vulnerability to pathologic stress responses in the setting of circadian misalignment, providing potential avenues of prevention and treatment using chronobiological strategies.