What Causes Seasonal Affective Disorder?

Seasonal Affective Disorder (SAD) is not simply a reaction to colder weather or shorter days. It is a form of depression influenced by biological, environmental, and psychological factors that interact with seasonal changes. Understanding what causes SAD can help individuals recognize symptoms early and seek effective treatment.

According to the National Institute of Mental Health (NIMH), SAD is considered a subtype of major depressive disorder that follows a predictable seasonal pattern, most commonly beginning in fall or winter, though summer-pattern SAD also occurs.

Reduced Sunlight and Circadian Rhythm Disruption

One of the primary contributors to seasonal affective disorder is reduced exposure to natural sunlight. Seasonal changes in daylight can disrupt the body’s internal clock, or circadian rhythm, which regulates sleep, mood, and hormone release.

The U.S. National Library of Medicine (MedlinePlus) explains that circadian rhythm disruption can lead to difficulty sleeping, low energy, and mood instability, all of which are core symptoms of depression.

When the body’s internal timing is out of sync, emotional regulation becomes more difficult.

Serotonin Imbalance

Serotonin is a neurotransmitter that plays a major role in mood stabilization. Seasonal changes in light exposure may reduce serotonin activity in the brain, contributing to depressive symptoms.

According to NIMH, lower serotonin levels are associated with increased risk of depression, including seasonal forms. Reduced sunlight may affect how serotonin is produced or used, increasing vulnerability during certain times of the year.

Melatonin and Sleep Changes

Seasonal shifts can also affect melatonin, a hormone that regulates sleep and wake cycles. Longer nights and reduced daylight may cause melatonin levels to rise or remain elevated longer than normal, leading to excessive sleepiness or difficulty maintaining consistent sleep patterns.

MedlinePlus notes that changes in melatonin production can worsen fatigue, concentration problems, and emotional instability associated with SAD

Genetic and Biological Risk Factors

Some individuals are more biologically predisposed to developing seasonal affective disorder. Research referenced by NIMH suggests that people with a family history of depression or bipolar disorder may be more vulnerable to seasonal mood changes.

SAD is also more common in people who live farther from the equator, where seasonal differences in daylight are more pronounced.

Environmental and Psychological Triggers

Seasonal changes often bring shifts in routine, activity level, and social interaction. Less time outdoors, reduced physical activity, and increased isolation can all contribute to depressive symptoms.

The U.S. Department of Health and Human Services Office on Women’s Health notes that stress, emotional vulnerability, and environmental factors can intensify depression, particularly when combined with biological risk factors

Why Understanding the Cause Matters in Recovery

For individuals in recovery, understanding the underlying causes of seasonal affective disorder is critical. Sleep disruption, mood instability, and low motivation can increase stress and weaken coping skills. When SAD goes unrecognized, it may increase the risk of relapse or mental health setbacks.

Identifying seasonal triggers allows treatment teams to adjust care plans proactively and support long-term stability.

Treatment and Support at Overland IOP

Seasonal Affective Disorder is highly treatable. Effective care often includes psychotherapy, medication when appropriate, structured routines, and targeted interventions that address sleep and mood regulation. Treatment plans should always be individualized, especially for those managing co-occurring mental health and substance use disorders.