In late January, researchers published the results of their study that seem to call into question whether seasonal affective disorder (SAD) actually exists. Seasonal affective disorder is a type of clinical depression that appears to be related in some way to the changing of the seasons (primarily winter and summer).
The new study contradicts dozens of previous studies that have found evidence that seasonal affective disorder does exist. So how do we square the results of the new study with the previous studies?
Seasonal affective disorder (SAD) is a kind of clinical depression that occurs during a particular time of year — usually either the onset of summer, or the onset of winter. Some have associated SAD with the shorter amounts of daylight that accompany the beginning of winter. But SAD also affects a smaller group of people during the onset of summer. The researchers write, “The idea that depression occurs along with seasonal changes or worsens in winter appears to be a well entrenched folk theory,” citing 993,000 Google hits on “winter depression.”
Seasonal affective disorder has — for a long time — been recognized as a legitimate, real disorder in the reference manual for mental disorders (the DSM-5). For a disorder to make it into the DSM-5, there has to be sufficient research that demonstrates the disorder is unique and can be reliably identified by a set of distinct signs or symptoms. SAD has been in the DSM since 1987 (nearly 20 years) as a modifier to a depressive episode.
The new study, published in Clinical Psychological Science, calls the very existence of SAD into question (Traffanstedt, Mehta & LoBello, 2016). They, along with other researchers, have long argued that SAD may not be a useful construct, even as a modifier for depression:
Historically, major depression with seasonal variation has been entangled with the SAD construct, and SAD research has relied to a large extent on the SPAQ. The SPAQ has little overlap with DSM depression criteria and is vulnerable to recall and other forms of bias [because it asks respondents about their depressive episodes over the past year].
The new study’s design was simple. The researchers looked at an existing set of data collected a decade ago from the Behavioral Risk Factor Surveillance System (BRFSS), a phone-based health survey conducted annually. The survey incorporates a standardized depression measure, which asks participants how many days in the previous 2 weeks they’ve experienced symptoms of depression. They gathered a nationally-representative sample of 34,294 adults, 1,754 of whom who would’ve been considered eligible for a diagnosis of clinical depression.
The researcher’s expectation is that if SAD is real, this telephone survey would find higher rates of depression during the onset of winter and summer — the times that SAD would seem to be most prevalent (given its definition).
Here’s how the study’s results were described on the journal publisher’s website:
Using geographic location for each participant, the researchers also obtained season-related measures including the actual day of the year, the latitude, and the amount of sunlight exposure.
The results showed no evidence that symptoms of depression were associated with any of the season-related measures. That is, people who responded to the survey in the winter months, or at times of lower sunlight exposure, did not have noticeably higher levels of depressive symptoms than those who responded to the survey at other times.
Problems with SAD Research
The problem with past research into SAD is that the primary measurement used doesn’t use the same depressive symptom criteria that is in the DSM — it uses a looser, less definitive set. This makes a lot of the previous SAD research suspect.
But the problem with this latest study is that it isn’t powerful enough to measure a disorder that is very rare in the population. We don’t really know the rate of SAD in the population, but if it occurs at a very low “base rate” (that is, is not commonly found even with the population of people with depression), research has to be specifically designed to detect it.
As the current researchers note, their research would not detect SAD if it occurred at such a low base rate:
“For example, Blazer et al. (1998) in an analysis of National Comorbidity Study data found that 1.6% of the major depression cases (representing 0.3% of the general population), reported seasonally related recurrent depression episodes.”
The current research wasn’t design to find SAD at such low rates. To their credit, the researchers mention this limitation of their research, but also seem to dismiss it as a legitimate alternative explanation for their findings.
But it seems odd that the researchers don’t believe that exposure to sunlight (or lack thereof) couldn’t impact a person’s mood. Certainly there is a pile of research demonstrating the impact of sunlight on people’s moods (e.g., Kadotani et al., 2014; Durvasula, et al. 2010; Serrano Ripoll, et al., 2015; Benedetti, et al., 2001; Umhau, et al., 2013; etc.). It isn’t much of a stretch to see how a person’s depression could also be connected to sunlight.
What researchers should now do is to try and replicate previous SAD research experiments with a more rigorous and DSM-consistent depression test, similar to the one used in the present study. If those replications fail, I suspect we’ll have our definitive answer.
This study won’t be the last word on whether SAD exists or not. For the hundreds of thousands of people who’ve been diagnosed and suffered from the disorder, it’s unlikely to sway their minds from the fact that it appears to be very real.
For further information
Psych Central: Is Seasonal Depression a Myth?
Benedetti, et al. (2001). Morning sunlight reduces length of hospitalization in bipolar depression. Journal of Affective Disorders, 62, 221-223.
Durvasula, S. et al. (2010). Sunlight and health: Attitudes of older people living in intermediate care facilities in southern Australia. Archives of Gerontology and Geriatrics, 51, e94-e99.
Kadotani, H., Nagai, Y., & Sozu, T. (2014). Railway suicide attempts are associated with amount of sunlight in recent days. Journal of Affective Disorders, 152-154, 62-168.
Serrano Ripoll., et al. (2015). Lifestyle change recommendations in major depression: Do they work? Journal of Affective Disorders, 183, 221-228.
Traffanstedt, M.K., Mehta, S., & LoBello, S.G. (2016). Major Depression With Seasonal Variation: Is It a Valid Construct? Clinical Psychological Science. doi: 10.1177/2167702615615867
Umhau, et al. (2013). Low vitamin D status and suicide: A case-control study of active duty military service members. PLoS ONE, 8. ArtID: e51543