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.
References:
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)
