Introduction: Shift Work Disorder (SWD) is a circadian rhythm sleep disorder in which there is a misalignment between an individual's circadian rhythm and work/sleep schedule. Accordingly, a questionnaire that screens for SWD would be optimally validated by its sensitivity to circadian phase markers. The present study attempts to determine whether the SWD Screening Questionnaire (SWDSQ), a 4-item questionnaire created in 2012 which can be used to classify individuals as HIGH RISK or LOW RISK for SWD, is correlated with circadian rhythm. Methods: 25 subjects (age: 35+/-8.9 years, 19F) who work 10-12 hour shifts between 1900 and 0800 participated in a 25 hour sleep deprivation session, during which they were kept in dim light < 10 lux). 9 asymptomatic individuals (ANW) and 16 individuals with SWD (Epworth Sleepiness Scale (ESS) >/= 10 and/or Insomnia Severity Index (ISI) >/= 10), who were free of other sleep disorders, participated in the study. Subjects completed the SWDSQ upon arriving for the study at 1630. Circadian rhythm was determined by salivary melatonin assay every 30 minutes for the duration of the study. The threshold for dim light melatonin onset (DLMO) was determined for each individual melatonin profile, and the return to baseline (DLMO OFF) occurred when melatonin amplitude fell and stayed below the threshold for at least two samples. Results: There is a significant correlation between SWDSQ classification and DLMO (r=0.52, p=0.01), DLMO OFF (r=0.59, p<0.01), and the duration between these two points (r=0.45, p=0.04). There is also a significant difference between the mean DLMO of the HIGH RISK group and the LOW RISK group (2000+/-5.9hrs vs. 0237+/-5.4hrs, p=0.01). Conclusion: The SWDSQ is sensitive to circadian phase, thus further validating its utility in screening for SWD.
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