Habitual light exposure relative to circadian timing in delayed sleep-wake phase disorder

Study Objectives: To compare melatonin timing, a well-validated marker for endogenous circadian phase, and habitual light-exposure patterns in adults with delayed sleep-wake phase disorder (DSWPD) and intermediate chronotype controls. Methods: Twelve individuals with DSWPD (five females, mean age: 31.1) and 12 age-matched controls (six females, mean age: 33.6) underwent a minimum of 7 days of light and activity monitoring followed by an inpatient hospital stay, where blood was taken to assess melatonin timing (calculated as dim light melatonin onset-DLMO). Habitual light-exposure patterns were then compared with a human phase-response curve (PRC) to light. Results: Relative to clock time, individuals with DSWPD had a later light-exposure pattern compared with controls, but their light-exposure pattern was earlier relative to DLMO. According to the human PRC to light, individuals with DSWPD had less daily advancing light exposure compared with controls. The primary difference was seen in the late portion of the advancing window, in which individuals with DSWPD were exposed to fewer pulses of light of equivalent duration and intensity compared with controls. Conclusions: Diminished advancing light exposure may play a role in the development and perpetuation of delayed sleep-wake timing in individuals with DSWPD. Enhancing light exposure during the later portion of the advancing window represents an innovative and complementary strategy that has the potential to improve the effectiveness of bright light therapy in DSWPD.