The researchers found the association only for those women who had very disrupted sleep patterns, having an actual light or TV left on in the room for most of the night. It is probably more likely to be the multitude of other health and lifestyle factors associated with this that are behind the weight gain – most likely poor diet and physical activity patterns. Then if there is a direct link it may be more likely that it's disturbed sleep that could affect metabolic processes and so predispose to weight gain, rather than the light itself.