Neuroscientists believe that even having a dim light on – such as a night light often used in a child’s room – adversely affects the chemical balance and structure of the brain.
Such a light appears to interfere with secretion of the hormone melatonin, which helps let the body know it is night time and time to sleep.
A team at Ohio State University in the US came to their conclusions after comparing two sets of Siberian hamsters, one group which was exposed to a dim light at night, the other which enjoyed complete darkness.
Tracy Bedrosian, a doctoral student who co-authored the study, said: “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.”
For example, she said they drank less sugar water.
When they examined the hamsters’ brains they found those exposed to dim night light had less dense networks of dendritic spines in a part of the brain called the hippocampus. Dendritic spines are the hairlike growths on brain cells that transmit chemical messages from one cell to another.
Bedrosian, who presented the research on Wednesday at the annual meeting of the American Society for Neuroscience in San Diego, added: “The hippocampus plays a key role in depressive disorders, so finding changes there is significant.”
Earlier studies in mice have found that those exposed to bright light at night tend to become depressed and put on weight.
Prof Randy Nelson, a neuroscience expert, said the results suggested more attention needed to be focused on how artificial lighting affects emotional health in people. “The increasing rate of depressive disorders in humans corresponds with the increasing use of light at night in modern society. Many people are now exposed to unnatural light cycles, and that may have real consequences for our health.
“The ability to escape light seemed to quell the depressive effects,” said Laura Fonken, another researcher. “But constant light with no chance of escape increases depressive symptoms.”