By Janet Raloff
Psychiatrists sometimes prescribe light therapy to treat a form of depression in people who get too little morning sun. But too much light at other times may actually trigger such mood disorders. Chronic exposure to light at night unleashes depression, a new study finds — at least in animals.
The new data confirm observations from studies of people who work night shifts, says Richard Stevens of the University of Connecticut Health Center in Farmington. Mood disorders join a growing list of problems — including cancer, obesity and diabetes — that can occur when light throws life out of balance by disrupting the biological clock and its timing of daily rhythms.
In the new study, appearing online July 24 in Molecular Psychiatry, Tracy Bedrosian, Zachary Weil and Randy Nelson of Ohio State University exposed Siberian hamsters to normal light and dark cycles for four weeks. For the next four weeks, half of the animals remained on this schedule, and the rest received chronic dim light throughout their night.
Compared with animals exposed to normal nighttime darkness, those getting dim light at night lost their intense preference for sweet drinks, “a sign they no longer get pleasure out of activities they once enjoyed,” Bedrosian says.
In a second test, animals were clocked on how long they actively tried to escape a pool of water. Hamsters exposed to night lights stopped struggling and just floated in the water — a sign of “behavioral despair” — 10 times as long as animals that had experienced normal nighttime darkness, Bedrosian reports.
An examination of tissue from the hippocampus — a brain structure that plays a role in depression — showed that animals that got light at night sported fewer nerve-cell protrusions known as dendritic spines. These structures mark sites of communication between cells. Such spine reductions “are consistent with what we see in humans with major depression,” Bedrosian says.
All symptoms of depression, including the nerve-cell changes, disappeared within two weeks of the animals returning to a normal light-dark cycle, the researchers report.
The scientists also could quash the behavioral symptoms — though not the drop in dendritic spines — by injecting the brains of animals with a drug that inhibits the activity of a molecule called tumor necrosis factor. Because this inflammatory chemical has been linked with human depression, this finding further suggests that light at night may trigger something akin to depression.
Human studies linking nighttime light and mood disorders are important but can’t easily probe molecular underpinnings as animal studies can, says George Brainard of Thomas Jefferson University’s Medical College in Philadelphia. The new work, he says, suggests that the disruption of the biological clock by light at night can be “an extremely potent force in regulating biology and behavior.”
