The Effect of Acute Bright Light Exposure on Social Affiliation

In recent decades, bright light has emerged as a useful tool in numerous clinical and non-clinical applications, with the potential to influence circadian rhythms, sleep, mood, and several other functional domains. However, despite the existence of plausible neurological pathways through which light could also influence social behavior, little is known at this point about the direct effects of bright light exposure on social interaction. Accordingly, the present study - utilizing a sample of young adults endorsing at least mild seasonal fluctuations in clinically relevant domains - examined the acute effects of a single 45-minute session of bright white light (15,000 lux) versus dim red light (200 lux) exposure on affiliative behavior. A significant interaction was observed between light condition and prior retinal sunlight exposure in the prediction of affiliative desire. Specifically, among study participants unexposed to high levels of morning blue-wavelength sunlight prior to the experiment, those in the bright light condition preferred the company of a stranger (another study participant) at a significantly higher level than did those in the control condition. In fact, they were nearly 6 times more likely than those in the dim red condition to elect such affiliation while awaiting a stressful speech task. No such between-group differences were observed among the subset of participants who, through nonadherence to the study protocol, were previously exposed to morning sunlight. Overall, these findings support the hypothesis that bright light exposure carries the potential to enhance affiliative drive, perhaps via cerebral serotonergic mediation. The results also raise the possibility that this salubrious alteration of social behavior may account for some of the established therapeutic effects of light therapy