Man-made nighttime lights have appeared everywhere in industrialized countries over the last century.What people are now asking is whether exposure to lightened darkness can affect the brain. To find out,Tracy Bedrosian,a doctoral student in neuroscience at Ohio State University,placed hamsters into two environments.In one,they were exposed to 16 hours of daylight and eight hours of complete darkness each day.In the other,the animals experienced 16 hours of daylight,but at nighttime,a dim light was kept on,at about the intensity of a TV screen lighting a dark room. After eight weeks,the researchers tested the hamsters for behavior that would suggest they were depressed.For example,they looked to see whether the hamsters still engaged in activities they normally enjoy,such as drinking sugar water. In people,loss of  enjoyment is a major symptom of depression. Hamsters in both groups were given a choice between drinking tap water or sugar water.The hamsters exposed to light at night drank similar amounts of tap and sugar water----they'd lost their preference for the sweet treat. These changes in behavior were associated with changes in a brain region known as the hippocampus  .The hamsters exposed to night light had a reduced number of so-called dendritic spines on the surface of cells in this region.These spines are hair-like things that stick out,and that brain cells use to communicate with one another. The findings match up with studies on humans that have d the hippocampus to be involved in depression. A patient with major depression has a smaller hippocampus. The brain changes in the hamsters might arise from fluctuations   in the production of the hormone melatonin  . Melatonin signals to the body that it's nighttime,but a light at night dampens its production. The hormone has been shown to affect mood,and so a decrease in melatonin might prompt depression,Bedrosian says. If the same mechanism is at work in people,then "people might want to try to avoid falling asleep with their TVs on at night,or to try to minimize light exposure during the night," she adds. According to the passage,one's mood is strongly affected by  _  .
the environment the spine the hormone melatonin brain cells