A new study on hamsters shows that a small amount of light while sleeping increased depressive symptoms. The light, equivalent to a what might be emitted by a small TV, affected the hippocampus region of the brain, which were thought to be the cause of the observed increased depressive symptoms.
Light entering closed eyelids at night alters melatonin production, which is an important hormone that regulates our sleep cycle, and now believed to effect the hippocampus as well.
Other sleep studies have shown that light exposure at night, such as from a night light, outside street lamps, or other sources, can negatively effect the quality of our sleep.
It's best to use black out curtains and ensure your room is dark enough that you can't clearly see your own hand held at arms length in front of your face. If you can see your hand, you room is too light for quality sleep.
Light at night causes changes in brain linked to depression