Neuroscientists believe that having even a dim night light while sleeping can disturb a person’s mental health and cause depressive symptoms.
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According to Tracy Bedrosian, co-author of the study and doctoral student in Neuroscience at Ohio State University, "Even dim light at night is sufficient to provoke depressive-like behaviors in hamsters, which may be explained by the changes we saw in their brains after eight weeks of exposure."
Previous studies in mice have shown that those exposed to bright light at night became depressed and obese.
Source-Medindia