If you get depressed when winter sets in, the reason could be your genes, a new study suggests.  Researchers say they’ve found link between a gene mutation and the common mood disorder.


Seasonal Affective Disorder, or SAD is one of the world's most common mood disorders, affecting approximately 3 percent of Americans and as many as 9 percent of the population in countries of far northern and southern latitudes, where the shortening of winter days is more extreme.

It is not yet known why some people are particularly sensitive to the shortening winter days, but the brain's light-sensitive circadian clock, which is our body’s natural timekeeper, is thought to be a factor.

Previous research has pointed to a link between sleep and depression more generally: Patients suffering from major depression often begin to sleep much more or less than usual, while disturbed sleep can also be a risk factor for developing depression.

So the UC San Francisco research team, which focuses on people with unusual sleeping patterns, looked at various mutated genes that are involved in causing Familial Advanced Sleep Phase (FASP), which causes people to go to bed as early as 7:30 p.m. and often waken by 4:30 a.m.

Now, by studying three members of a family in which many members experience both FASP and SAD, they have identified a specific gene mutation that occurs in family members who experience both conditions, suggesting that the gene – PER3, a member of the Period family of circadian genes – is apparently the link between sleep and mood.

They also introduced the mutated form of the gene into genetically engineered mice and simulated the effects of changing seasons by altering the length of "day" and "night" mice experienced in the lab.  Remarkably, the mutant mice behaved in many ways like humans with FASP, appearing quite normal when day and night were equal, but shifting their circadian rhythms and developing mild depression-like symptoms when exposed to 4-hour days and 20-hour nights.

"It's an exciting time," said Ying-Hui-Fu, PhD, one of the lead authors. "People have known for decades that light and mood were linked, but this is our first real finger-hold on the problem,” he added of the research, which appears in Proceedings of the National Academy of Science.