Seasonal affective disorder is major depressive disorder with a seasonal pattern. Most commonly, depression begins in late autumn as daylight hours shorten and remits in spring as daylight returns. The pattern must be reproducible across at least two consecutive years for the seasonal specifier to apply. Less commonly, the pattern reverses with summer depression (related to heat sensitivity and disrupted sleep).
The pathophysiology is rooted in circadian biology. The suprachiasmatic nucleus of the hypothalamus orchestrates the body's 24-hour rhythm via photic input — light hitting the retina signals the SCN to align internal time with external time. With shortened daylight in winter, this entrainment weakens. Downstream effects include altered melatonin secretion timing, shifted serotonin signaling (serotonin transporter binding is upregulated in winter in SAD patients on imaging studies), and altered dopaminergic and cortisol rhythms.
Prevalence varies with latitude. SAD is rare at the equator and increasingly common at higher latitudes. In Nordic countries and northern North America, prevalence can reach 5-10%. Sub-syndromal seasonal mood changes are even more common. Female predominance is consistent.
Phenotype often includes atypical features: increased appetite (particularly carbohydrate craving), weight gain in winter, hypersomnia rather than insomnia, fatigue, decreased social engagement. The atypical presentation is part of why SAD patients often don't recognize it as depression — they describe it as "winter blues" or "hibernation mode."
Light therapy is first-line treatment. Standard protocol: 10,000 lux light box for 30 minutes daily in the early morning, ideally within an hour of waking, starting in autumn before symptoms develop and continuing through winter. Effective in roughly 60-80% of patients, often within 2-4 weeks. Side effects are mild (occasional headache, eye strain). Therapeutic light boxes deliver the specific intensity and spectrum needed to entrain the SCN — different from regular indoor lighting.
SSRIs and SNRIs are also effective. Bupropion XL has FDA approval specifically for preventing seasonal depression — started in autumn before symptoms emerge. Combination therapy (light + SSRI) for severe cases. CBT-SAD addresses behavioral patterns and cognitions specific to seasonal depression.
Practical considerations: vitamin D supplementation has limited evidence for SAD treatment but is appropriate for documented deficiency common in northern winters. Sleep hygiene, exercise, outdoor light during daylight hours all support treatment. Travel to lower latitudes can dramatically improve symptoms — sometimes the most diagnostic move. When you encounter winter-onset depression with reproducible pattern, the seasonal specifier applies. Light therapy is remarkably effective, costs little, and produces meaningful improvement in most patients.