Researchers at Ohio State University have found that the presence of light during sleep negatively affects the chemical balance and structure of the brain. Even dim light, such as a night light, interferes with the secretion of melatonin, the hormone that induces sleep.

After comparing two sets of hamsters, the researchers concluded that those sleeping in dim lighting exhibited depressive-like behaviour, as compared to those sleeping in complete darkness. This, they believe, is a result of changes that occurred in the brains after eight weeks of sleeping in dim lighting.

After the eight week experimental period, researchers found that the hamsters in the dim-lighting group had less dense networks of dendritic spines in the hippocampus, a region of the brain that plays a central role in depressive disorders. Dendritic spines are hair-like growths that are responsible for transmitting chemical messages between brain cells.

This study adds to previous findings connecting exposure to light during sleep and depressive behaviour. One study found that mice exposed to bright lights at night tend to become depressed and to gain weight.

The findings suggest that light at night constitutes a disruption of the body's natural sleep cycle, and that this disruption can have significant effects on mental health. The researchers noted that, given the fact that even dim lighting produced such an effect, people should be wary of the level of lighting when they sleep.