Seasonal affective disorder (SAD) is a mood disorder with depressive symptoms that occur at a specific time of year with full remission at other times of year. It typically occurs during fall or winter, although a less common form occurs during spring or summer.^1,2 The pathophysiology is unclear, but theories include circadian rhythm disruption, dysregulation of the melanopsin signaling pathway and its impact on serotonin reuptake, and dysfunction of the hypothalamic-pituitary-adrenal axis.^3–5 Studies using interviews and diagnostic criteria in the United States, Canada, and United Kingdom show a lifetime prevalence of 0.5% to 2.4% in the general population.^6–8 Of patients with major depression, 10% to 20% have a seasonal pattern of symptoms consistent with SAD.⁹

What Are the Risk Factors for SAD?

Evidence for risk factors is mostly weak or limited but suggests that family history, female sex, living at a more northern latitude, and young adulthood (18 to 30 years of age) may increase risk.^10–15

EVIDENCE SUMMARY

An Australian study of 4,000 twins compared symptoms in identical twins with symptoms in fraternal twins. Sets of identical twins were twice as likely to have SAD than sets of fraternal twins, suggesting a possible genetic link.¹⁰ Specific associated genes have not been identified, but a 2018 meta-analysis revealed ZBTB20 as a potential susceptibility gene for SAD.¹¹

Two large questionnaire studies have shown that individuals living in more northern latitudes have an increased risk of developing SAD.^12,13 It is suspected that prevalence increases with increasing latitude, but data are poor, and this conclusion is controversial.^12,13

Two studies using the Seasonal Pattern Assessment Questionnaire (SPAQ) found that SAD has a higher prevalence in females, but this is based on poor data.^14,15 A 2000 meta-analysis of population studies found that SAD appears to be most prevalent among young adults.⁹

What Are the Diagnostic Criteria for SAD?

In addition to meeting Diagnostic and Statistical Manual of Mental Disorders , 5th ed., (DSM-5) criteria for major depressive disorder or bipolar disorder, there must be a temporal relationship between time of year and occurrence of mood episodes. Mood episodes are discrete periods during which the patient demonstrates depressive symptoms. Full remission must occur when the specified season ends. There must be two consecutive years of mood episodes in the same season to diagnose fall/winter or spring/summer SAD. In SAD, seasonal mood episodes significantly outnumber nonseasonal mood episodes during the individual's life.¹

EVIDENCE SUMMARY

The DSM-5 does not define SAD as a separate diagnosis and instead includes it as a variant of major depressive disorder or bipolar disorder.¹ Typical features of depression include depressed mood, anhedonia, guilt or hopelessness, decreased energy, and impaired mood. Typical features of bipolar disorder include mania, hypomania, depression, irritability, anxiety, mood lability, sleep disturbance, and hyperactivity. SAD is a subtype or qualifier of these mood disorders with a seasonal onset and remission.

The more common fall/winter type of SAD has occasional atypical features, such as increased need for sleep, carbohydrate cravings with increased appetite and weight gain, and extreme fatigue.¹⁶ The less common spring/summer type may have the atypical symptoms of increased irritability, poor appetite with weight loss, insomnia, agitation, restlessness, anxiety, and increased violence.¹⁷

To meet the diagnostic criteria, psychosocial stressors of a specified season, such as layoffs during the winter season, must not contribute to the mood episodes. Additionally, the diagnosis requires a two-year consecutive pattern of mood episodes during a specified season.¹