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 significantly influence autoimmune disease activity, with colder months triggering more flares and inflammation in genetically susceptible people. A growing body of research reveals that environmental seasonality doesn't just affect how you feel—it actually reshapes your immune system's behavior at the molecular level, influencing everything from which immune cells are active to how your genes are expressed.

How Does Winter Change Your Immune System?

During winter months, your immune system undergoes a dramatic shift. Research shows that winter triggers a proinflammatory profile in your body, characterized by elevated levels of C-reactive protein (CRP) and interleukin-6 (IL-6) receptor—both markers of inflammation. While this winter immune response likely evolved to help your ancestors fight off seasonal infections, it has an unintended consequence: it lowers the threshold for autoimmune inflammation in people who are genetically vulnerable.

A pivotal 2024 pilot study provided direct evidence of how seasonal changes reshape specific immune cells. Researchers compared immune profiles in 29 women between late summer and late winter in Hobart, Australia, and found a higher percentage of regulatory T cells (Tregs)—immune cells that help calm inflammation—in summer at 7.3% compared to just 7% in winter. Naive Tregs, a particularly important subset, were 13% higher in adult women during summer. Additionally, women who spent more hours outdoors in summer had significantly lower levels of Th17 cells, a type of immune cell that drives inflammation.

Which Autoimmune Diseases Are Most Affected by Seasonal Changes?

A comprehensive 2025 analysis examining data from 201 countries identified a striking pattern: autoimmune diseases are significantly more prevalent in colder climates. The research revealed that type 1 diabetes (T1D) showed the strongest association with cold temperatures, with approximately 48% of the variation in disease prevalence across countries explained by average annual temperature. This suggests that environmental factors linked to cold climates—such as reduced sunlight, vitamin D deficiency, and higher rates of respiratory infections—create a permissive environment for autoimmunity.

Beyond type 1 diabetes, several other autoimmune conditions show clear seasonal patterns:

• Inflammatory Bowel Disease (IBD): Shows strong associations with colder regions, with patients frequently experiencing disease exacerbations during winter months.
• Psoriasis: Patients commonly report worsening symptoms in winter, likely due to reduced ultraviolet radiation, lower humidity, and shifts in circadian rhythms that affect skin immune responses.
• Rheumatoid Arthritis: Exhibits a moderate thermal association with cold climates, and patients frequently report symptom worsening during colder months due to seasonal changes in immune gene expression and epigenetic regulation.

What Role Does Vitamin D Play in Seasonal Autoimmune Flares?

One of the most studied mechanisms linking winter to autoimmune disease is vitamin D deficiency. Autoimmune patients often have insufficient vitamin D levels, defined as concentrations below 20 nanograms per milliliter (ng/mL). The active form of vitamin D, called 1,25(OH)2D3, works by suppressing inflammatory immune cells called Th1 and Th17 cells while simultaneously boosting regulatory T cells—the immune cells that help prevent your body from attacking itself.

However, the relationship between vitamin D supplementation and autoimmune disease improvement is more nuanced than simply taking more supplements. Clinical trial data shows mixed results: some studies report reduced relapse rates or reduced need for treatment escalation in people with deficiency, while others demonstrate no significant benefit. Experts recommend that supplementation should be individualized and primarily aimed at correcting actual deficiency rather than routine high-dose use in people who already have adequate vitamin D levels.

Can Your Birth Season Predict Autoimmune Disease Risk?

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

Scientists propose two main mechanisms to explain this phenomenon. The vitamin D hypothesis suggests that winter gestation results in maternal vitamin D deficiency, which affects fetal thymic development—the thymus is the organ where immune cells learn to distinguish between self and non-self. The viral infection hypothesis proposes that perinatal exposure to seasonal viruses triggers autoimmunity through a process called molecular mimicry, where viral proteins resemble the body's own proteins, confusing the immune system.

How Do Your Body's Internal Clocks Influence Seasonal Immune Changes?

Beyond sunlight and temperature, your body's circadian rhythm—your internal 24-hour biological clock—plays a crucial role in seasonal immune regulation. Approximately 23% of the human genome, including critical immune genes, exhibits significant seasonal variation in expression. Circadian clock genes, particularly one called BMAL1, have been implicated in autoimmune regulation. Research in experimental lupus models demonstrates that when BMAL1 is deficient in myeloid immune cells, autoantibody production and immune complex deposition increase, suggesting that disrupted circadian rhythms may worsen lupus severity.

Melatonin, a light-regulated neurohormone whose secretion peaks at night, also plays a role in seasonal immune variation. This hormone exerts antioxidant and immunomodulatory effects; however, its effects in autoimmune diseases such as rheumatoid arthritis appear mixed and context-dependent, meaning the benefit may vary depending on individual circumstances.

What Should People With Autoimmune Disease Do With This Information?

Understanding seasonal patterns in autoimmune disease opens the door to more personalized, chronobiology-based care—medical approaches that account for your body's natural seasonal and circadian rhythms. Monitoring your disease activity across seasons could help you and your healthcare provider anticipate flares and adjust treatment proactively rather than reactively. For example, if you notice your symptoms consistently worsen in winter, your doctor might recommend preventive strategies during fall months, such as vitamin D level testing and optimization, increased outdoor light exposure, or adjustments to your medication regimen.

The research also highlights why seasonal factors matter beyond just feeling worse: seasonal changes interact with your genetic and epigenetic mechanisms—the chemical switches that turn genes on and off without changing your DNA sequence—to shape your autoimmune vulnerability across your entire lifespan. This means that seasonal awareness isn't just about managing current symptoms; it's about understanding a fundamental biological pattern that influences your long-term health trajectory.