Seasonal Affective Disorder (SAD) is more than “winter blues.” For many people it’s a recurrent, predictable pattern of mood change that can seriously disrupt work, relationships, appetite, sleep, and motivation. This article explains why symptoms spike at certain times of year, who’s at risk, and evidence-based strategies — practical, step-by-step — to reduce symptoms and regain functioning.

What is SAD?

SAD is a subtype of major depressive disorder (or, less often, a bipolar pattern) characterized by depressive episodes that recur seasonally — most commonly beginning in autumn and winter and resolving in spring. Symptoms include low mood, hypersomnia, increased appetite and carbohydrate cravings (often with weight gain), low energy, social withdrawal, and difficulty concentrating. Less commonly, some people experience summer-onset SAD with opposite features (insomnia, agitation, weight loss).

Why symptoms spike in certain months — the science, simply explained

1. Reduced daylight alters circadian timing.
   Shorter days in autumn/winter change the timing of the body’s internal clock (circadian rhythm). When your circadian system becomes misaligned with the local light–dark cycle, sleep and mood regulation can be disrupted.
2. Melatonin changes — more sleepiness, less alertness.
   Melatonin (the “sleep” hormone) is produced in the dark. Longer nightly darkness can increase melatonin secretion or shift its timing, contributing to excessive sleepiness and lethargy.
3. Serotonin activity may drop.
   Serotonin turnover in the brain has been observed to be lower in winter months in susceptible people; reduced sunlight is one factor that can lower serotonin function and contribute to depressive symptoms.
4. Photoperiod sensitivity and latitude effects.
   The likelihood and severity of SAD rise with latitude: the farther from the equator, the bigger seasonal shifts in daylight, and the higher the SAD rates. Photoperiod (day length) appears to be a key trigger for people who are sensitive to these changes.
5. Behavioral and social factors.
   Colder, darker months often bring reduced outdoor activity, less socializing, increased sedentary time, and dietary shifts — all of which can worsen mood. Holiday stress and disrupted routines can also compound vulnerability.

When do symptoms typically spike?
In the northern hemisphere....

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