The causes of depression are many and complex, but who'd have thought too much light could be part of the problem? Hamsters who sleep in total darkness are less susceptible to depression, and the same could be true of humans.
Ohio State neuroscientist Tracy Bedrosian placed hamsters in one of two different environments. In the first, the hamsters spent 16 hours in daylight and then spent 8 hours in absolute darkness. The second group still got 16 hours of daylight, but their 8 sleeping hours were spent in the company of a dim light, roughly the equivalent of leaving one's TV on all night in an otherwise dark room.
Eight weeks later, Bedrosian tested the hamsters for signs of depression. The easiest way to spot depression in an animal like a hamster is see whether they demonstrate a preference for pleasurable treats like sugar water. The hamsters that spent 8 hours in total darkness showed a clear preference for sugar water over regular water, but the hamsters that slept with a dim light showed no preference at all. That suggests they're suffering from anhedonia, or an inability to feel pleasure, a key indicator of depression.
Indeed, there were key differences in the brains of the two hamster groups. The hamsters that slept with the light had fewer dendritic spines, structures found near the hippocampus that are crucial for communication between brain cells. This ties in with human studies of depression that show the disease correlates with a shrinking of the hippocampus region.
Bedrosian suggests that these brain changes could be caused by a deficiency in the hormone melatonin. The hormone is essential in letting the body know that it's nighttime, but nearby light sources can reduce its production. Other studies have revealed melatonin's antidepressant qualities, which suggests a lack of it could spur on depression. This also fits neatly with studies demonstrating an increased risk of mood disorders for people who work at night.
Right now, this effect has only been demonstrated in hamsters, not humans. Still, there's decent reason to think this could hold true for humans as well, so Bedrosian says it might be a good idea to avoid falling asleep with the TV or other light sources on. If nothing else, turning these off before you go to sleep will save on electricity bills, and that should be enough to buoy anyone's spirits.