When people become depressed during the winter months, it is not the result of reduced exposure to sunlight, according to new research published last week in the journal Clinical Psychological Science.
In fact, the study found no significant correlation between depression and season, latitude or sunlight exposure.
These findings call into question the whole notion of seasonal affective disorder (SAD), a type of depression whose symptoms are described as having a seasonal pattern, usually worsening in the fall and winter and then remitting in the spring and summer.
Seasonal depression has been included in the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders (DSM) since 1987.
“In conversations with colleagues, the belief in the association of seasonal changes with depression is more or less taken as a given and the same belief is widespread in our culture,” said Steven LoBello, the study’s senior author and a professor of psychology at Auburn University at Montgomery, in a released statement. “We analyzed the data from many angles and found that the prevalence of depression is very stable across different latitudes, seasons of the year, and sunlight exposures.”
This study’s results — if true — should not be interpreted as meaning that people who experience depression during the fall and winter months are imagining their symptoms. All these findings suggest, write LoBello and his colleagues in their study, is that “merely being depressed during winter is not evidence that one is depressed because of winter.”
Although most people are unaware of it, studies that have examined the claim that seasonal changes are behind some recurrent episodes of major depression have had conflicting results. Part of the problem, says Lobello and his co-authors, is that researchers often ask people to recall when they were depressed, a method of gathering data that is vulnerable to inaccuracies and bias. People who have heard of SAD, for example, may tend to remember their symptoms as developing as the days became shorter, whether they did or not.
For their study, Lobello and his colleagues decided to use data that captured symptoms of current depression in a large group of people. They then analyzed that data to see if it was related to measures of sunlight exposure.
The data came from 34,294 American adults, aged 18 to 99, who participated in the 2006 Behavioral Risk Factor Surveillance System (BRFSS) survey. The survey, which is conducted annually by the Centers for Disease Control and Prevention, included a list of questions that have been validated as a tool to measure current depression. A total of 1,754 people gave answers to those questions that met the criteria for depression.
The BRFSS data also included the date and geographic location for each of the survey’s interviews, which allowed Lobello and his colleagues to determine the latitude where the participants lived and (using U.S. Naval Observatory information) the amount of sunlight that participants were exposed to at the time of their interview.
After using several statistical models to analyze the data, the researchers found that season, latitude and sunlight exposure had no significant effect on symptoms of depression. Those findings held even after adjusting for a variety of possible confounding factors, such as age, race, gender, educational level, marital status and employment status.
“The findings cast serious doubt on major depression with seasonal variation as a legitimate psychiatric disorder,” Lobello and his colleagues write.
Limitations and implications
This study has several limitations. Most notably, participants were not diagnosed clinically — by a doctor — but only as a result of answers they gave in a phone interview. Some of the participants may not have answered those questions truthfully, particularly since admissions of depression often carry a social stigma.
It’s also possible that people with clinical depression may have not answered the phone at all.
This study is not able to prove — or disprove — that clinical depression has a seasonal variation. But, argue Lobello and his co-authors, if that variation does exist, it most likely affects only a very small subset of people.
“Depression is a recurrent illness,” they write. “Because all episodes of depression occur in some season, chance occurrence in two consecutive winters would explain some apparent seasonality. The role of chance as an explanation diminishes in cases where episodes are experienced in three or more consecutive winters. Even so, the existence of such cases would not in themselves demonstrate that changes in sunlight exposure are responsible for the depression.”
“The weight of accumulating evidence, including the evidence presented here, indicates that the burden of proof for including the seasonal variation modifier for major depression in DSM has shifted to those who would continue to do so,” they conclude.