Research in animal experiments shows that light at night is a risk factor for both weight gain and depression. This probably involves an interruption of natural processes involving melatonin. Melatonin agonists are now being used for the treatment of depression.
Chronic dim light at night provokes reversible depression-like phenotype: possible role for TNF. (Link)
Circadian rhythms in the Zucker obese rat: assessment and intervention. (Link)
Light at night increases body mass by shifting the time of food intake. (Link)