During times of disease, like flu season, we have long known that people become more apprehensive in crowded locations where the chances of being infected are higher. Architects who design the lighting for public spaces have also known that dim lighting is favoured for places that are going to become quite crowded, like bars and theatres. Fascinated by this, psychologists Chen-Bo Zhong and Ping Dong at the University of Toronto set out to explore whether dim illumination reduces nervousness about the risk of infection. They found out that it did.
The team theorised that darkness increases our sense of distance from one another and that this, in turn, makes us view threats associated with human contact as somehow less relevant. Over the course of two initial experiments the researchers found that participants staying in a dimly lit room or wearing sunglasses estimated a lower risk of catching contagious diseases from others than did those staying in a brightly lit room or wearing clear glasses. In follow up experiments they even found that the effect persisted in both lab and real-life settings.
You can read more about this research in Psychological Science.