Seasonal Affective Disorder (SAD)

Seasonal Affective Disorder describes a pattern in which major depressive episodes recur at the same time each year — most commonly during fall and winter months, with full remission by spring or summer — and is understood to be closely linked to reduced sunlight exposure and its effects on the body's internal circadian rhythm. A much less common summer pattern also exists, presenting differently with agitation and insomnia rather than the more typical winter 'slowing down' pattern.

SAD is not a separate diagnosis in DSM-5, but rather a 'seasonal pattern' specifier applied to Major Depressive Disorder (or, less commonly, Bipolar Disorder), defined by a regular temporal relationship between the onset of depressive episodes and a particular time of year, full remission (or in bipolar disorder, a shift to mania/hypomania) at a characteristic time of year, this pattern being demonstrated over at least two consecutive years, and seasonal episodes substantially outnumbering any non-seasonal episodes across the person's lifetime.

Reduced exposure to natural sunlight during shorter winter days is thought to disrupt the circadian rhythm regulated by the brain's suprachiasmatic nucleus, altering the timing of melatonin secretion and affecting serotonin transporter activity, which tends to be higher (reducing available serotonin) during winter months. This explains the greater prevalence of winter-pattern SAD at higher latitudes, where winter daylight hours are especially limited.

• Living at higher latitudes, further from the equator
• Family history of SAD or other mood disorders
• Female sex
• Younger age
• History of other mood disorders
• Occupations involving limited daylight exposure (indoor work, night shifts)

Reduced winter light exposure is thought to cause a 'phase delay' in melatonin secretion timing, disrupting the alignment between the internal circadian clock and the external light-dark cycle. Additionally, seasonal changes in serotonin transporter density affect how much serotonin remains available for mood regulation, with transporter activity peaking (and available serotonin dropping) during darker winter months — providing a biological basis for both the seasonal symptom pattern and the effectiveness of light therapy.

• Winter pattern (most common): low mood, low energy and fatigue, increased sleep (hypersomnia), increased appetite with carbohydrate cravings, weight gain, and social withdrawal
• Summer pattern (less common): agitation, insomnia, decreased appetite, weight loss, and increased anxiety
• Difficulty concentrating
• Loss of interest in usually enjoyable activities during the affected season

• A regular temporal relationship between the onset of major depressive episodes and a particular time of year (typically winter)
• Full remission (or a shift to mania/hypomania in bipolar disorder) also occurs at a characteristic time of year
• This pattern has occurred for 2 or more consecutive years
• Seasonal depressive episodes substantially outnumber any non-seasonal depressive episodes over the person's lifetime

Conditions a psychiatrist will consider and rule out before confirming this diagnosis:

Non-seasonal Major Depressive Disorder

Lacks the consistent, recurring seasonal timing pattern that defines SAD.

Bipolar Disorder with seasonal pattern

Screening for any history of hypomania or mania (sometimes emerging in summer) is important before treating with antidepressants alone.

Hypothyroidism

Can independently cause fatigue and weight gain; thyroid function should be checked regardless of the seasonal pattern.

Other causes of hypersomnia

Conditions such as sleep apnea should be considered if excessive sleepiness is a prominent, persistent feature.

• Clinical interview tracking mood patterns across seasons and years
• Seasonal Pattern Assessment Questionnaire (SPAQ)
• Thyroid function tests
• Sleep assessment where hypersomnia or sleep-disordered breathing is suspected

Bright light therapy — typically 10,000 lux for 20–30 minutes each morning — is the first-line, specifically evidence-based treatment for winter-pattern SAD. Antidepressant medication, particularly extended-release bupropion (specifically approved for SAD prevention), and CBT adapted for seasonal depression (CBT-SAD) are also effective, and treatments are often started proactively before the expected seasonal onset.

Bupropion (extended-release)

Specifically approved for the prevention of seasonal major depressive episodes, typically started in early fall.

SSRIs

Effective for seasonal depressive episodes, similar to their use in non-seasonal MDD.

Light therapy (bright light box)

First-line treatment for winter-pattern SAD, used each morning, ideally starting proactively before the season's typical symptom onset.

CBT-SAD

A version of cognitive behavioral therapy specifically adapted for seasonal depression, addressing seasonal thought patterns and behavior (e.g., social withdrawal, reduced activity).

• Maximize natural morning light exposure, such as a walk outside shortly after waking
• Use a light therapy box consistently each morning during the risk season, as prescribed
• Maintain regular exercise, even when motivation is reduced
• Keep a consistent sleep-wake schedule year-round
• Plan engaging activities during darker months rather than allowing social withdrawal to take hold
• Discuss vitamin D levels with your doctor, as deficiency is common in affected individuals, though evidence for vitamin D alone as a treatment is limited

SAD responds well to light therapy and/or medication, and because the pattern is predictable, proactive treatment starting before the usual seasonal onset can often prevent or substantially reduce the severity of the next episode.

• Starting light therapy or medication proactively in early fall, before symptoms typically begin
• Maintaining consistent exercise and social engagement through the darker months
• Considering extended light exposure strategies (light boxes, dawn-simulating alarms) as a preventive routine each year

Understand SAD as a real, biologically based condition tied to seasonal light changes, not simply 'winter blues' or a lack of motivation. Encourage consistent use of light therapy, since regular daily use matters more than occasional use, support morning outdoor time, and watch for the pattern returning each year so treatment can start early rather than after symptoms are already established.

1. American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, 5th Edition, Text Revision (DSM-5-TR) — seasonal pattern specifier.
2. National Institute of Mental Health (NIMH). Seasonal Affective Disorder.