What color is your — or your child's — nightlight? It's fairly well established that exposure to blue light at night leads to difficulty in getting to sleep because it lowers melatonin production.

Blue light can also affect mood and set the stage for depression, according to a recent Ohio State University study. Better to go for something at the red end of the spectrum.

Considering how many light sources people today are exposed to at night, the best news may come from another study by the same researchers…

In the study, which was done on hamsters, exposure to dim light at night of other colors, especially blue, led to symptoms of depression.

Adult female Siberian hamsters were exposed to four weeks each of nighttime conditions with no light, dim red light, dim white light or dim blue light. Researchers then checked the hamsters for depressive symptoms.

Hamsters, like many people, have a sweet tooth and when given sugar water as a treat, will lap it up. Ignoring sugar water when it's offered is taken as a sign of depression. After the four-week study, hamsters exposed to dim blue or white light drank the least sugar water. Those that were kept in the dark drank the most, closely followed by those exposed to red light.

The researchers then looked for changes in the hippocampal region of the brains of the hamsters. And the hippocampi of hamsters that spent the night in dim blue or white light had significantly fewer dendritic spines than those that spent the night in total darkness or in red light. Dendritic spines are hairlike growths on brain cells that are used to send chemical messages from one cell to another. A decrease in dendritic spines has been linked to depression.

In fact, in nearly every measure of mood looked at, hamsters exposed to blue light were the worst off, followed by those exposed to white light. While total darkness was best, red light was not nearly as bad as light of other colors.

Results found in hamsters don't always predict what will happen in people. But the researchers do believe that their findings may be meaningful for people. And if they are, they may prove even more important for shift workers than for nightlight users. Several studies have found links between shift work and depression and other mood disorders. Shift workers, who usually work in light much brighter than a nightlight, might benefit substantially from a switch to red light.

Considering how many light sources people today are exposed to at night, the best news may come from another study by the same researchers which found that the depression caused in hamsters by four weeks of dim white light at night disappeared two weeks after the nighttime light exposure ended.

The study appears in the Journal of Neuroscience.