A reader asks:

I experience advanced sleep phase syndrome (ASPS) in addition to SAD. My understanding of SAD is to treat it with morning light. My understanding of ASPS is to treat it with evening light. Which time of the day should I schedule my 30-minute light box treatment?


This is a terrific question. For our readers, advanced sleep phase syndrome, (ASPS) is a circadian rhythm sleep phase disorder in which one falls asleep unusually early (say, 8 p.m.) and wakes up unusually early (say, 3 a.m.). To set the circadian clock to a later hour, light therapy has to be taken around the start of the “subjective night” (say, 8 p.m.), because morning light could shift the sleep cycle even earlier. On the other hand, light therapy for SAD is most effective at the end of the subjective night (say, 6:30 a.m. for a normal sleeper), and even those with a strong morning chronotype benefit most from morning light. (For a determination of your chronotype, complete the Morningness-Eveningness Questionnaire at www.cet.org.)

The chronotype of people with ASPS falls outside the range of normal chronotypes, and their antidepressant response to morning vs. evening light has not been adequately investigated. This may be a case where treatment should be scheduled in both morning AND evening, as was the original formula for SAD treatment. The specific timing of the two treatment sessions would need to be determined by a specialist, with the goal of delaying the sleep episode while maintaining morning exposure (in the case above, at, say, 6:30 a.m.). One technological means to implement such a schedule is with a dusk-to-dawn simulator used to straddle the sleep interval in the bedroom. A high-intensity dusk signal could be followed by a lower intensity dawn signal, to achieve an optimum balance. We definitely need more research on such applications.