Behavioral engagement level, variability, and diurnal rhythm as a function of bright light in bipolar II seasonal affective disorder: an exploratory study.

Six patients with bipolar II seasonal affective disorder (SAD) and seven normal control subjects rated their moods in winter at six fixed times each week-day during 1 week of dim and 2 weeks of bright light. The scales rated represent the mood dimension specifically associated with depression, a dimension here called behavioral engagement (BE). Compared with controls, depressed SAD subjects (1) showed lower BE levels across all rating times of the day, (2) were more likely to show diurnal variation in BE, (3) displayed more between-day instability in BE diurnal rhythm, and (4) exhibited greater short-term lability (change within 3 hours) in BE. Bright light reduced or eliminated all group differences in BE level and variability.