As the days grow shorter and cold, and darkness settles in, some begin to feel a little blue -- hamsters and people alike.
Up to 20 percent of Americans report they feel more depressed during the winter months as a result of a condition known as seasonal affective disorder. Now scientists have shown that hamsters experience the same sluggishness when their exposure to light is reduced. By studying these sad hamsters, the researchers hope to find new ways of helping people combat seasonal depression.
"Hamsters aren't perfect models, but the mechanisms may be similar -- as in what's going on in the brain that causes seasons to change emotional behavior," explained Leah Pyter, a graduate student in neuroscience at Ohio State University in Columbus.
How do you know when a hamster is depressed? Pyter explained there were a few tests established in laboratory science that claimed to determine the mood of the rodents.
For example, if the animals spend more time hovering near the walls of their containers, rather than at the center, it's believed they feel more anxious. If they decline to slurp up tempting offers of sugar water, scientists take it as a sign of depression. Another test involves placing the animals in water and seeing if they swim or simply give up and float. Hamsters don't sink apparently, but float in water.
"The sooner they give up in the water, the more depressed they are," Pyter said. "If you give them an antidepressant they don't give up as quickly."
In her research, led by OSU's Randy Nelson, 53 female and 48 male Siberian hamsters were housed in containers simulating either long summer days (16 hours of light) or short winter days (eight hours of light). After about 60 days in their fluorescent-lit environments the hamsters were tested to measure their level of anxiety and depression. Pyter and Nelson found a clear link between the amount of light exposure and the hamsters' state of minds.
"Our results do suggest a relationship between season and symptoms of depression and anxiety," Nelson said.
The researchers then went further and tested whether the animals' exposure to light in the days after their birth had an impact later in life. Studies in people have suggested that those born during the winter months may be more vulnerable to depression. Sure enough, Pyter and Nelson found that baby hamsters born to short artificial days were more likely to show signs of depression and anxiety later in life.
"Animals born in short days still had some depressive behaviors even when they were exposed to longer days as adults," Pyter said. Nonetheless, she added that the amount of light the animals were exposed to as adults seemed to have more influence on mood than the amount of light they were exposed to as baby hamsters.
For those of us in the Northern Hemisphere, the shortest day of the year is fast approaching. Dec. 21 is winter solstice when the sun shines directly over the Tropic of Cancer and then sinks from the sky sooner than all other days of the year. How does the sun's path affect our mood? Norman Rosenthal, author of "Winter Blues: Everything You Need to Know About Seasonal Affective Disorder," says it may be linked to a time when humans had no access to artificial light.