Seasonal Affective Disorder: A Functional Medicine Approach Beyond the Light Box

Yoon Hang Kim MD

Board-Certified in Preventive Medicine | Integrative & Functional Medicine Physician

About Dr. Kim

Dr. Yoon Hang Kim is a board-certified preventive medicine physician with over 20 years of experience in integrative and functional medicine. A University of Arizona/Andrew Weil Center fellowship-trained physician, he holds certifications in preventive medicine, medical acupuncture, and integrative and holistic medicine. Dr. Kim specializes in low dose naltrexone (LDN), autoimmune conditions, chronic pain, integrative oncology, fibromyalgia, chronic fatigue syndrome, mast cell activation syndrome (MCAS), and mold toxicity.

He is the author of eight books and more than 25 peer-reviewed articles.

Professional: www.yoonhangkim.com | Clinical: www.directintegrativecare.com

Important: This article is for educational purposes only and does not constitute medical advice, diagnosis, or treatment. Always consult your healthcare provider before making changes to your care plan. The information here is not a substitute for a professional clinical evaluation.

Seasonal Affective Disorder: A Functional Medicine Approach Beyond the Light Box

Every year, as the days shorten and the light fades, millions of people feel something shift. Not just a preference for staying in—but a heaviness that settles into the bones. Sleep becomes excessive but never refreshing. Cravings for carbohydrates intensify. Motivation drains away. For many, this isn’t just “winter blues.” It’s seasonal affective disorder—SAD—and it’s a real, recurring form of depression.

What I find striking after two decades of practice is how often SAD gets reduced to a single intervention—buy a light box, take some vitamin D, wait it out. That approach isn’t wrong, exactly, but it misses the bigger picture. And in functional medicine, the bigger picture is the whole point.

Why SAD Deserves a Deeper Look

Conventional medicine correctly identifies that reduced daylight disrupts circadian rhythms, serotonin signaling, and melatonin production. These are real mechanisms. But here’s the question I ask my clients: why does the same seasonal change devastate one person and barely affect another?

In a functional medicine framework, we look at the terrain—the individual vulnerabilities that make a person susceptible. That means asking about gut health, nutrient status, inflammation, hormonal balance, sleep architecture, toxic burden, and stress resilience. SAD doesn’t happen in a vacuum. It happens in a body that may already be running on fumes.

Light Therapy Still Matters—Here’s How to Do It Right

Let me be clear: light therapy is an evidence-based, first-line treatment for winter-pattern SAD, and I use it regularly in my practice. A 10,000 lux white-light box used for 20–30 minutes each morning can start shifting mood within days to weeks. The key details that get overlooked:

•  It must be UV-filtered white light—not a tanning lamp, not a decorative “happy light.”

•  Timing matters. Morning exposure anchors your circadian rhythm. Evening use can backfire.

•  Distance from the light box determines actual lux at your eyes. Follow the manufacturer’s positioning instructions.

•  Consistency trumps intensity. Daily use throughout the symptomatic season matters more than occasional long sessions.

•  If you have a history of bipolar disorder, retinal problems, or photosensitizing medications, talk with your clinician first. Light therapy can trigger mood elevation or mania in susceptible individuals.

The Functional Medicine Layer: What I Look for Beyond Light

When a client comes to me with recurrent SAD, light therapy and psychotherapy are part of the plan—but they’re not the whole plan. Here’s where a functional medicine approach adds depth:

Vitamin D: Correct the Deficiency, Don’t Chase the Cure

Low vitamin D levels are common in people with SAD, and that correlation has led to a lot of enthusiasm. But here’s the honest truth: supplementation studies for SAD are inconsistent. Vitamin D is not a proven antidepressant.

That said, correcting a documented deficiency is still important. Vitamin D plays roles in immune modulation, inflammation, and neurotransmitter synthesis. When levels are low, I check 25-hydroxyvitamin D and replete accordingly. I don’t present it as a SAD cure—I present it as part of optimizing the terrain. And excessive doses can be toxic, so this is always done with lab monitoring.

The Gut-Brain Axis: Where Mood Really Lives

Roughly 90–95% of the body’s serotonin is produced in the gut. If intestinal permeability, dysbiosis, or chronic GI inflammation are present, the raw materials for mood regulation may be compromised before seasonal darkness even enters the picture. This is one of the most underappreciated contributors to treatment-resistant mood disorders.

In my practice, I’ll often run a comprehensive stool analysis, organic acids testing, or food sensitivity panels when a client’s SAD doesn’t respond to standard approaches. Sometimes the answer isn’t more light—it’s healing the gut.

Inflammation and Immune Dysregulation

Chronic low-grade inflammation is emerging as a driver of depression across the board, not just seasonal patterns. Inflammatory cytokines can directly impair serotonin and dopamine pathways. For clients with autoimmune conditions, mast cell activation syndrome, mold exposure, or chronic infections, the inflammatory burden may be the primary reason they’re more vulnerable to SAD.

Addressing inflammation—through dietary interventions, targeted anti-inflammatory support, and identifying hidden triggers—can shift the landscape significantly.

Melatonin, Circadian Rhythm, and Sleep Architecture

Melatonin has a small evidence base for sleep support in SAD, but it’s not a general-purpose antidepressant. What I find more useful clinically is looking at the entire circadian framework: when is the client eating, sleeping, exercising, and getting light exposure? Circadian disruption amplifies every other SAD vulnerability. Restoring rhythm—consistent wake times, morning light, time-restricted eating—is free and profoundly undervalued.

Methylation, B Vitamins, and Neurotransmitter Support

B12, folate, and B6 are essential cofactors in serotonin, dopamine, and norepinephrine synthesis. But routine supplementation without testing is guesswork. A small B12 trial did not demonstrate benefit for SAD specifically, and blanket B-vitamin use isn’t supported by the literature.

What is supported is checking. If a client has MTHFR variants or functional methylation issues, targeted support becomes individualized and meaningful—not a shot in the dark. This is the difference between functional medicine and supplement stacking.

A Word on Omega-3s, SAMe, and St. John’s Wort

These come up constantly, so let me be direct:

•  Omega-3 fatty acids have some general depression data, but SAD-specific evidence is insufficient. They’re not a primary treatment. They may play a role in an anti-inflammatory protocol.

•  SAMe is sometimes used for depression, but SAD-specific evidence is inadequate. Medication interactions are real—especially with antidepressants. This one needs clinician oversight.

•  St. John’s wort carries major drug interaction risks. It induces cytochrome P450 enzymes, which can reduce the effectiveness of oral contraceptives, blood thinners, immunosuppressants, and many other medications. Combined with serotonergic drugs, it raises the risk of serotonin syndrome. Small SAD studies are inconclusive. I rarely recommend it because the risk profile is significant.

Cognitive Behavioral Therapy and Medication: Still Essential

I want to be honest about something. Functional medicine is powerful, but it isn’t a replacement for cognitive behavioral therapy or antidepressant medication when those are indicated. CBT adapted for SAD helps clients identify seasonal thinking traps and behavioral withdrawal patterns. It teaches resilience tools that carry forward year after year.

When depression is moderate to severe, medication may be necessary—and there’s no shame in that. SSRIs are commonly used, and for clients with predictable recurrent SAD, bupropion has the strongest prevention evidence. An integrative approach isn’t about rejecting pharmaceuticals. It’s about making sure we’re also addressing what made the body vulnerable in the first place.

Building Your Integrative Plan

If you’re dealing with recurrent winter depression, here’s the framework I use with my clients:

1.  Get a proper clinical assessment. Make sure it’s actually SAD and not another form of depression, bipolar disorder, or thyroid dysfunction masquerading as seasonal symptoms.

2.  Start with evidence-based treatments. Morning light therapy, CBT for SAD, and medication if severity warrants it.

3.  Investigate the terrain. Check vitamin D, inflammatory markers, B vitamins, thyroid function, and gut health. Correct what’s off.

4.  Rebuild circadian rhythm. Consistent sleep-wake times, morning outdoor light when possible, movement, and structured eating patterns.

5.  Address inflammation. Anti-inflammatory nutrition, identifying hidden triggers, and for some clients, targeted interventions based on their unique inflammatory profile.

6.  Use supplements judiciously. Test, don’t guess. Targeted repletion based on lab work is medicine. Grabbing five bottles off a shelf is not.

The Bottom Line

Seasonal depression is real depression, and it deserves real treatment. Light therapy works. CBT works. Medication works. And a functional medicine approach—investigating why your body is vulnerable in the first place—can make all of those treatments work better.

The goal isn’t to replace proven interventions with supplements. It’s to stop treating the surface and start asking deeper questions. That’s the essence of what I do.

If you’re struggling with seasonal depression—especially symptoms that are severe, worsening, or affecting your ability to function—please reach out for a professional evaluation. You don’t have to muscle through this alone.

Work with Dr. Kim

Dr. Kim offers membership-based integrative and functional medicine consultations via telemedicine. If you’re interested in a root-cause approach to seasonal depression, mood disorders, or chronic conditions, visit www.directintegrativecare.com to learn more about his practice, or visit www.yoonhangkim.com for professional consulting inquiries.

References

1. Lam RW, Levitt AJ, Levitan RD, et al. Efficacy of bright light treatment, fluoxetine, and the combination in patients with nonseasonal major depressive disorder: a randomized clinical trial. JAMA Psychiatry. 2016;73(1):56–63.

2. Rosenthal NE, Sack DA, Gillin JC, et al. Seasonal affective disorder: a description of the syndrome and preliminary findings with light therapy. Arch Gen Psychiatry. 1984;41(1):72–80.

3. Melrose S. Seasonal affective disorder: an overview of assessment and treatment approaches. Depress Res Treat. 2015;2015:178564.

4. Penckofer S, Kouba J, Byrn M, Estwing Ferrans C. Vitamin D and depression: where is all the sunshine? Issues Ment Health Nurs. 2010;31(6):385–393.

5. Yano JM, Yu K, Donaldson GP, et al. Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell. 2015;161(2):264–276.

6. Berk M, Williams LJ, Jacka FN, et al. So depression is an inflammatory disease, but where does the inflammation come from? BMC Med. 2013;11:200.

7. Modell JG, Rosenthal NE, Harriett AE, et al. Seasonal affective disorder and its prevention by anticipatory treatment with bupropion XL. Biol Psychiatry. 2005;58(8):658–667.

8. Rohan KJ, Mahon JN, Evans M, et al. Randomized trial of cognitive-behavioral therapy versus light therapy for seasonal affective disorder: acute outcomes. Am J Psychiatry. 2015;172(9):862–869.

9. Sarris J, Logan AC, Akbaraly TN, et al. Nutritional medicine as mainstream in psychiatry. Lancet Psychiatry. 2015;2(3):271–274.

10. National Institute of Mental Health. Seasonal Affective Disorder. https://www.nimh.nih.gov/health/publications/seasonal-affective-disorder. Accessed 2026.

Yoon Hang Kim MD is the author of MCAS: Epidemic in Plain Sight and LDN Primer, both available on Amazon. He is the founder of the LDN Support Group.