Seasonal Affective Disorder: An Overview

As SAD is frequently believed to be a mild condition, it is often overlooked by clinicians.
As SAD is frequently believed to be a mild condition, it is often overlooked by clinicians.

Seasonal affective disorder (SAD) is "recurring major depression with a seasonal pattern."1 Its prevalence is between 1% and 10% of the population and more typically affects women than men (with a ratio of 4:1).2

SAD is no longer considered a discrete diagnostic entity but is categorized by the Diagnostic and Statistical Manual of Mental Health Disorders, Fifth Edition (DSM-5) as a type of depression.3 Full remittance is necessary during other seasons, and the pattern must continue for at least 2 years, with more episodes of seasonal depression than seasons without depression over the course of a lifetime.3


"It is important to bear in mind that SAD is not just 'winter blues' but is an episode of major depression," Kelly Rohan, PhD, professor of psychological science at the University of Vermont, Burlington, emphasized to Psychiatry Advisor.

Seasonal Variations

The most common presentation of SAD is a winter seasonal pattern, with symptoms occurring during late fall and winter and remitting during spring and summer. However, there is also a summer seasonal pattern. While both conditions involve symptoms of irritability and suicidal thoughts, winter-type SAD more typically presents with sadness, frequent crying, fatigue and lethargy, somnolence, decreased activity levels, social withdrawal, carbohydrate cravings, and weight gain.1 Summer-type SAD, on the other hand, is more likely to present with poor appetite, weight loss, insomnia, agitation, and restlessness.1

"Triggers for winter depression concern the photoperiod, meaning the number of hours between sunrise and sunset, while individuals who experience summer SAD are sensitive to heat and humidity," Dr Rohan told Psychiatry Advisor.

Etiology and Pathophysiology

Several hypotheses have been proposed to understand the biological mechanisms underlying SAD, primarily the phase shift hypothesis, which focuses on circadian phase delay or advance.4 In winter-type SAD, this is presumed to mean there is a disturbance in the patient's biological clock, an "internal desynchronization between rhythms driven by the biological clock and the sleep-wake rhythm."2

Several factors may account for this dysfunction. One is that people with SAD may have difficulty in serotonin regulation. During the winter months, they overproduce SERT, a protein that transports serotonin from the synaptic cleft to the presynaptic neuron. Higher SERT levels cause diminished serotonin activity, thereby leading to depression. During the summer, sunlight keeps SERT levels low, but as sunlight wanes during the fall, SERT levels rise, serotonin levels fall, and depression increases.1

Individuals with SAD may also have difficulty with overproduction of melatonin, a hormone produced by the pineal gland that causes drowsiness in response to darkness.1 Increased darkness during the wintertime leads to increased sleepiness and lethargy.1

Decreased serotonin and increased melatonin affect circadian rhythms, which are synchronized to respond to daily rhythmic light-dark changes. The circadian signal of changes in day length appears to be timed differently in individuals with SAD, thereby impairing the ability of the body to adjust to seasonal changes.1

"The phase shift hypothesis states that in response to earlier dawns in winter, the biological clock may run slower in people vulnerable to winter depression, resulting in a state comparable to jet lag that does not go away, while the photoperiodic hypothesis focuses on the melatonin rhythm and the overproduction of melatonin in the winter as a result of the shorter period of daylight," Dr Rohan said.

Geographical location may play a role in SAD, with more northern latitudes evincing more cases. "This makes sense because the length of day gets shorter the further one gets from the equator," Dr Rohan explained. Outside the United States, however, only a trend to increased depression in higher latitudes has been found, with studies in northern European countries showing mixed results.5

Richard R. Schwartz, MD, associate professor at Harvard Medical School and senior consultant for residency training at McLean Hospital in Boston, Massachusetts, noted that there may be a genetic component. "Some research suggests that there are genetic variants of SAD that are connected with melanopsin receptors and melatonin," he told Psychiatry Advisor.

Diagnosing SAD

One of the screening tools used in the diagnosis of SAD is the Seasonal Pattern Assessment Questionnaire (SPAQ), a retrospective, self-administered questionnaire.6 However, the validity of the SPAQ has been called into question for potentially inflating the prevalence estimates of the condition.7

"The SPAQ is good at identifying people with a seasonal problem, but not at differentiating SAD from 'winter blues,' so the gold standard is the clinical interview, using DSM-5 criteria for major depression, and finding out whether those symptoms follow a seasonal pattern," Dr Rohan said.

Does SAD Really Exist?

Recent research has called into question whether SAD is indeed an actual entity.8 It has been noted that in countries such as Norway, which is without sun for two winter months, there appears to be no greater incidence of depression during the “dark period.”8 A 2016 study examined depression rates drawn from a database of 34,292 subjects using the 8-item Patient Health Questionnaire depression scale (PHQ-8) to investigate the presence or absence of depression.9 The results showed no evidence that depressive symptoms were associated with geographical or season-related measures.

"I think this was a problematic study," Dr Rohan said. "It looked at a national database of people with depression to see if there were more cases of depression during certain seasons or at certain latitudes and could not find this relationship. They concluded therefore that SAD does not exist."

"I think there is an entity of SAD," Dr Schwartz agreed. "The connection between depression and duration of daylight is clear, but a new kind of blurriness is emerging from recent studies that bright light and circadian interventions can treat nonseasonal as well as seasonal depression.10,11 This complicates the picture and the clear line between the two. But the evidence for a seasonal component of some people's mood is pretty convincing."

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