The relation between chronotype and treatment outcome with light therapy on a fixed time schedule

Background: Seasonal affective disorder (SAD) is characterized by recurrent episodes of major depression in a seasonal pattern. The therapy of choice is light therapy (LT). It is suggested that LT should be administered relative to the chronotype of the patient, with the optimal timing earlier for morning than for evening types. This study aims to retrospectively investigate the relation between chronotype and the effect of LT on a fixed time in the morning in a population of SAD patients.

Methods: Data from four different studies conducted at the University Center of Psychiatry in Groningen, the Netherlands was used. Data from 132 patients was used (103 women). Depression score was determined by a structured interview (SIGH-SAD) prior to LT and after LT. Prior to LT morningness/eveningness preference of the patient was determined by the 'Morningness/Eveningness Questionnaire' (MEQ). All patients received LT at 8:00 AM at the clinic, independent of chronotype.

Results: Patients had an average MEQ score of 51.5±8.2. There was no significant relationship between MEQ score and therapy success as measured with the SIGH-SAD (F2,129=0.05, ns). When patients were divided by chronotype (ranging from definite morning to moderate evening) no significant relation between MEQ score and therapy success was found (F2,129=0.02, ns).

Limitations: Retrospective design.

Conclusions: The lack of a significant relationship between chronotype, as measured with the MEQ, and therapy success with LT at a fixed timepoint may indicate that the anti-depressive effect of morning light in SAD patients is not explained by a phase shift of the biological clock.