The influence of light administration on interpersonal behavior and affect in people with mild to moderate seasonality

•Participants received light and placebo treatments each for 21 days in winter.•Mood and social behavior were measured using ecological momentary assessment.•Bright light improved mood, decreased submissiveness and increased quarrelsomeness.•A wrist monitor showed that 21% of participants did not adhere to light treatment.

Bright light is used to treat winter depression and may also have positive effects on mood in some healthy individuals. However, there is little information on how bright light treatment influences social behavior. We performed a cross-over study in winter comparing the effects of morning bright light administration with placebo (exposure to negative ions) on mood and social behavior in 38 healthy people with mild to moderate seasonality. Each treatment was given for 21 days with a washout period of 14 days between treatments. An event-contingent recording assessment was used to measure mood, and social behavior along two axes, agreeable-quarrelsome and dominant-submissive, during each 21-day treatment period. During treatments, participants wore a combined light-sensor and accelerometer to test this method for adherence to light treatment self-administered at home. Data were analyzed using multilevel modeling. Bright light improved mood but increased quarrelsome behavior and decreased submissiveness. Data from the light monitor and accelerometer suggested that 21% of the participants did not adhere to bright light treatment; when this group was analyzed separately, there was no change in quarrelsomeness or mood. However, results for individuals who followed the procedure were similar to those reported for the whole sample.