It makes sense that the human body is affected by the amount of daylight in their environment. Animals are defined, in part, by whether they sleep and wake in the day, night, or at dawn and dusk. It stands to reason that their body clocks receive signals from light; human beings are no different.
Many people notice certain SAD-like symptoms occurring in winter, even if the symptoms are not severe enough for them to have a diagnosis of Seasonal Affective Disorder. Symptoms of SAD like lethargy, increased food intake and carbohydrate craving and an increase in sleep often occur at that time. Research supports the idea that these winter blues symptoms are caused by light levels.
Studies on Seasonal Affective Disorder Symptoms and Light
One study by Dr. Michael Terman and others, found that across the lower half of the United States, the proportion of people with SAD increases linearly with latitude until a latitude of 38 degrees north. There was no further increase in the disorder in people in states further north of that line. The location of 38 degrees north latitude in the United States extends from Santa Rosa California across to Washington DC.
It has also been shown that 25% of people living at latitude 38 degrees and higher experience symptoms of Seasonal Affective Disorder. Not everyone’s symptoms are severe enough to be diagnosed with the disorder, but the symptoms are still prevalent.
The study also showed that people living on the western edge of time zones tended to have more Seasonal Affective Disorder than people living on the eastern edges of time zones. It is believed that this has something to do with the later time of sunrise relative to sleep. In other words, if people go to sleep at 10:00 p.m., but for those on the western edge of the time zone, the sun rises later in the morning than for those in the east, this is believed to cause more Seasonal Affective Disorder symptoms.
Other Risk Factors for Winter Blues: Age and Female Gender
Approximately half a million Americans are diagnosed with Seasonal Affective Disorder. Women make up between 60 and 90% of those with the disorder, but women are also at a higher risk for Major Depressive Disorder (MDD) so it is unclear whether or not gender is a risk factor for SAD over and above the MDD risk factor.
According to the DSM-IV, younger people are also more at risk. But according to a recent study by M. Terman and others, there was 23% more winter depression between ages 30 to 50 than at ages 20 to 30 or 50 to 60. This was true for both genders above the 38 degree north latitude.
Light Treatment for SAD
Seasonal Affective Depression is often treated with light therapy, which is proven to be effective. Light therapy is also used to treat sleep problems, shift workers, other depressive and bipolar disorders that are not seasonal, winter blues and adult ADHD, It is recommended that people consult a professional for treatment. Other treatment options are also available.
Many people experience SAD-like symptoms in winter at a sub-clinical level. The symptoms seem to be related to light, hormones and circadian rhythms. Research shows that where a person lives in relation to latitude, and in some cases, longitude is a risk factor. Age and gender also seem to be risk factors. Treatment with light and other treatment is effective and should be done in consultation with a professional.
Click this link to read the Interview with Dr. Michael Terman about Seasonal Affective Disorder
Sources:
- The American Psychaitric Association’s Diagnostic and Statistical Manaual of Mental Disorders, Fourth Edition (DSM-IV).
- Michael Terman, T.M. White, S. Fairhurst, G.H. Musa, Predictors of Winter Depression: Latitude and Longitude in Relation to Photoperiod and Sunrise, available at the Society for Light Treatment and Biological Rhythms.
- Columbia University Center for Light Treatment and Biological Rhythms.
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