Why Your Autoimmune Disease Gets Worse in Winter—And What Science Says About It

Seasonal changes in temperature, sunlight, and your body's internal clock directly influence autoimmune disease activity, with winter consistently triggering worse symptoms and higher disease prevalence worldwide. A groundbreaking 2025 study analyzing data from 201 countries found that colder climates show significantly higher rates of autoimmune diseases, suggesting that environmental seasonality plays a major role in how these conditions develop and flare.

How Winter Changes Your Immune System

Your immune system doesn't stay the same year-round. A 2024 pilot study provided the first direct evidence that specific immune cells shift dramatically between seasons. Researchers compared immune profiles in 29 women between late summer and late winter, finding that regulatory T cells (Tregs)—the immune cells that help calm inflammation—were 13% higher in summer than winter. Women who spent more hours outdoors in summer had significantly lower levels of inflammatory Th17 cells, suggesting that sunlight exposure directly influences immune balance.

During winter, your immune system shifts toward a pro-inflammatory state, meaning it becomes more prone to attacking your own tissues. Levels of inflammatory markers like C-reactive protein (CRP) and interleukin-6 (IL-6) increase during colder months. This winter immune shift likely evolved to help your body fight off seasonal infections, but in people with genetic susceptibility to autoimmune disease, it lowers the threshold for inflammation and flares.

Which Autoimmune Diseases Are Most Affected by Seasons?

Not all autoimmune conditions respond equally to seasonal changes. The 2025 international study revealed striking differences in how temperature influences different diseases:

• Type 1 Diabetes: Shows the strongest association with cold climates, with approximately 48% of the variation in disease prevalence across countries explained by average annual temperature alone
• Inflammatory Bowel Disease (IBD) and Psoriasis: Also exhibit strong associations with colder regions, with winter often triggering severe flares
• Rheumatoid Arthritis: Shows a moderate but significant thermal association, with patients frequently reporting worsening symptoms during winter months

Psoriasis patients, for example, commonly experience winter severity due to reduced ultraviolet radiation, lower humidity, and circadian rhythm disruptions that affect how skin cells behave and how the immune system responds.

The Vitamin D and Viral Infection Connection

Two major mechanisms explain why winter worsens autoimmune disease. First, reduced sunlight exposure leads to vitamin D deficiency. The active form of vitamin D suppresses inflammatory Th1 and Th17 cells while boosting regulatory T cells—exactly the opposite of what happens in winter. Many autoimmune patients have vitamin D levels below 20 nanograms per milliliter (ng/mL), considered insufficient, and studies show a correlation between low vitamin D and increased disease activity.

Second, winter brings a surge in respiratory viral infections like influenza. Severe flu infections can trigger immune hyperactivation and have been linked to autoimmune flares, including Guillain-Barré syndrome and type 1 diabetes onset. The proposed mechanism is "molecular mimicry," where viral proteins resemble your body's own proteins, confusing the immune system into attacking both the virus and your tissues.

Your Birth Season May Predict Autoimmune Risk

Interestingly, the season you were born in may influence your lifetime autoimmune disease risk. A 2022 systematic review of 11 studies found that 73% identified a significant seasonal pattern in birth months of patients who later developed autoimmune endocrine diseases like Hashimoto's thyroiditis. About 64% of studies found birth peaks in spring and summer, with the pattern more pronounced in women.

Two competing theories explain this. The "vitamin D hypothesis" suggests that winter pregnancy exposes the developing fetus to maternal vitamin D deficiency, affecting thymic development (the thymus is where immune cells mature). The "viral infection hypothesis" proposes that perinatal exposure to seasonal viruses triggers autoimmunity through molecular mimicry mechanisms.

What You Can Do: Practical Steps for Seasonal Management

Understanding seasonal patterns opens the door to proactive management. Experts recommend monitoring your disease activity throughout the year and working with your healthcare provider to anticipate winter flares. If you have vitamin D insufficiency, supplementation may help, though research shows mixed results—the goal should be correcting deficiency rather than routine high-dose supplementation in people with adequate levels.

Beyond vitamin D, consider lifestyle factors that support immune balance: maintaining consistent sleep schedules (since circadian rhythm disruption worsens inflammation), managing stress, and getting outdoor light exposure during winter months when possible. For conditions like rheumatoid arthritis and psoriasis, winter flares may warrant closer monitoring and earlier intervention with your rheumatologist or dermatologist.

"Patients with chronic or autoimmune conditions often feel stuck managing symptoms without understanding why they persist," explains Dr. David Noonan, a functional medicine provider. "This approach allows us to look deeper—using advanced diagnostics and a whole-person lens—to create care plans that are truly individualized. When patients receive consistent guidance and coaching, we often see meaningful improvements in both symptoms and quality of life".

The Future of Seasonal Autoimmune Care

Approximately 23% of the human genome, including critical immune genes, exhibits significant seasonal variation in expression. This discovery is reshaping how researchers think about autoimmune disease management. Rather than treating these conditions as static year-round problems, emerging evidence suggests that chronobiology-based care—adjusting treatment intensity and preventive strategies based on seasonal patterns—could improve outcomes while reducing medication burden.

The key takeaway: if you have an autoimmune disease and notice your symptoms worsen predictably in fall or winter, you're not imagining it. Your immune system genuinely shifts with the seasons, and understanding this pattern empowers you to work with your healthcare team to stay ahead of flares rather than simply reacting to them.