Circadian phase in delayed sleep phase syndrome: predictors and temporal stability across multiple assessments

STUDY OBJECTIVES: To assess temporal stability across multiple assessments and predictors of circadian phase in participants with delayed sleep phase syndrome (DSPS), relative to normal-sleeping matched controls. DESIGN: Circadian phase was assessed by salivary dim light melatonin onset (DLMO) during 3 laboratory visits, separated by at least 5 days--2 scheduled at the end of the week (Friday) and 1 scheduled at the end of the weekend (Sunday). PATIENTS: Eight young volunteers who met International Classification of Sleep Disorders-Revised criteria for DSPS, and 8 age- and sex-matched controls (age range 19-27 years old). INTERVENTIONS: N/A. MEASUREMENTS AND RESULTS: As expected, salivary DLMO occurred significantly later in patients with DSPS than in controls (F 10.561, p = .006). However, circadian phase did not change significantly across the 3 DLMO assessments in either group. Estimations of circadian phase were not significantly different in the assessments conducted on weekdays versus weekends. Predictors of circadian phase included time of morning light exposure (R2 = 0.777; p < .001), recent wake time (R2 = 0.701, p < .001), and self-reported chronotype (R2 = 0.320, p = .016). DLMO preceded wake time in both groups by approximately 10.75 hours. CONCLUSIONS: Across serial laboratory assessments on an ad lib sleep schedule, patients with DSPS appeared more similar to than different from normal-sleeping control subjects, except for a stable delay in circadian phase.