Exposure to even dim light at night, such as a glowing TV screen, could prompt changes in your brain that lead to mood disorders -- up to and including depression.
Researchers placed hamsters in sleep environments with different lighting conditions. After eight weeks, the hamsters were tested for behaviors that might suggest depression.
According to Live Science:
"The hamsters exposed to light at night showed behaviors indicative of anhedonia (depression-like response in which one does not find pleasure in favorite activities), and changes in the hippocampus of the brain.
If the same mechanism is at work in people, then [according to the researchers] 'people might want to try to avoid falling asleep with their TVs on all night, or they might want to try to minimize light exposure during the night'".