Doctors sometimes use 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 cause such mood disorder. Long-lasting exposure to light at night brings 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. Mood disorders join a growing list of problems, including cancer, obesity and diabetes that can occur when light throws life out of balance by disturbing the biological clock and its timing of daily rhythms. In the new study, Tracy Bedrosian and Randy Nelson of Ohio State University exposed mice to normal light and dark cycles for four weeks. For the next four weeks, half of the mice remained on this schedule, and the rest received continuous dim light throughout their night. Compared with mice exposed to normal night-time darkness, those getting dim light at night lost their strong preference for sweet drinks. "A sign that they no longer get pleasure out of activities they once enjoyed," Bedrosian says. In a second test, mice were clocked on how long they actively tried to escape a pool of water. Those exposed to night lights stopped struggling and just floated in the water, a sign of "behavioral despair", 10 times as long as the mice that had experienced normal night-time darkness. All symptoms of depression disappeared within two weeks of the mice returning to a normal light-dark cycle, the researchers report. The scientists could also _ the behavioral symptoms by injecting the brains of animals with a drug that prohibits the activity of certain molecules linked with human depression. This finding further suggests that light at night may cause something related to depression. Human studies linking night-time light and mood disorders are important but can not easily detect molecular underpinnings  as animal studies can, says George Brainard of Thomas Jefferson University. The new work, he says, suggests that the change of the biological clock by light at night can be "an extremely powerful force in regulating biology and behavior". After being exposed to continuous night-time light, the mice   _  .
changed their preferences escaped from the water more eagerly remained active as before showed less interest in their favorites