Can Neuroscience Help Explain Why Shorter Days Can Lead to Darker Moods?

The winter months bring shorter days and darker nights, but for some they also bring a darker mood. Seasonal affective disorder (SAD), a type of seasonal depression commonly known as “winter blues” affects one in five people. To be diagnosed with SAD, people must meet full criteria for major depression coinciding with specific seasons (appearing in the winter or summer months) for at least 2 years. Seasonal depressions must be much more frequent than any non-seasonal depressions.

People with SAD have difficulty regulating the neurotransmitter serotonin, a neurotransmitter believed to be responsible for balancing mood. Scientists are trying to identify the causes of SAD. Two recent studies suggest that the culprit is a brain circuit that connects special light-sensing cells in the retina with brain areas that affect whether a person feels happy or sad. These researchers believe that these cells detect shorter days; they appear to use this pathway to send signals to the brain that can make a person feel glum or even depressed.

Dr. Jerome Sanes, a professor of neuroscience at Brown University, was part of a team that found evidence of the brain circuit in people. A few weeks earlier, a different team published a study suggesting a very similar circuit in mice.”It’s very likely that things like seasonal affective disorder involve this pathway,” observed Dr. Sanes.

According to the National Institute for Mental Health (NIMH), these attributes may increase your risk of SAD:

• Being female. SAD is diagnosed four times more often in women than men.
• Living far from the equator. SAD is more frequent in people who live far north or south of the equator. For example, 1 percent of those who live in Florida have SAD versus 9 percent of those who live in New England or Alaska.
• Family history. People with a family history of other types of depression are more likely to develop SAD than those who do not have a family history of depression.
• Having depression or bipolar disorder. The symptoms of depression may worsen with the seasons if someone has one of these conditions (but SAD is diagnosed only if seasonal depressions are the most common).
• Younger Age. Younger adults have a higher risk of SAD than older adults. SAD has also been reported in children and teens.

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