2026-01-06
Seasonality and mental health: how winter and summer shape our minds
Psychiatry
By Carolina Lima | Published on January 6, 2026 | 3 min read
Seasonal changes have long been suspected to influence mood and psychiatric disorders. Clinical observations confirm that symptoms fluctuate throughout the year, yet the underlying mechanisms remain unclear. A recent review published in the Journal of Clinical Medicine (2025) by Modzelewski and colleagues introduces the immunoseasonal theory, offering an insightful immunological perspective on how weather patterns impact mental health.
Depression tends to peak during autumn and winter, a period marked by low sunlight, cold temperatures and high humidity. These conditions coincide with increased antidepressant use and a higher risk of postpartum depression in late-year births. Anxiety disorders and post-traumatic stress disorder share similar inflammatory pathways making them equally vulnerable to winter exacerbations.
Schizophrenia shows a different seasonal pattern: hospitalizations frequently rise in summer, with some research noting a secondary peak in winter. Factors such as light exposure and melatonin regulation may play a role in modulating immune responses in these patients.
Bipolar disorder also demonstrates clear seasonality, with manic episodes surging in summer and early autumn, while depressive episodes cluster in the colder months.
The immunoseasonal theory proposes that seasonal fluctuations in psychiatric symptoms are driven by shifts in immune system activity, particularly the balance between T-helper 1 (Th1) and T-helper 2 (Th2) responses.
These immune shifts do not act in isolation. They interact with neurotransmitters such as serotonin and dopamine, disrupt circadian rhythms and amplify stress pathways through HPA axis dysfunction. They also alter brain network connectivity, creating seasonal vulnerability: winter favors depression and anxiety, while summer increases the risk of psychosis, mania, and impulsive behaviors.
Weather strongly influences immune balance and psychiatric risk.
In winter, cold temperatures and high humidity promote viral infections, amplifying Th1-driven inflammation and worsening depression and anxiety.
Summer heat and allergens shift immunity toward Th2 dominance, increasing vulnerability to psychosis, mania, and impulsive behaviors. Heatwaves and extreme temperatures correlate with higher hospitalizations for schizophrenia and mood disorders. Sunlight generally improves mood but can temporarily raise suicide risk in spring.
Other factors, such as low atmospheric pressure and strong winds, show modest links to impulsivity and psychiatric admissions, though evidence remains inconsistent.
If this theory holds true, it could change how we approach mental health care. Care could become more personalized based on the season by adjusting medication doses, adding anti-inflammatory or anti-allergic strategies and using biomarkers like IL-6 or Th1/Th2 ratios to predict risk.
Still, much of the evidence is observational. We need long-term studies, standardized methods, and tools like neuroimaging and circadian rhythm assessments to confirm these links. Understanding how seasons shape our minds could help us move from reactive care to proactive prevention—making psychiatry smarter, more precise, and truly patient-centered.
About the Author – Carolina Lima
Carolina is a specialist in Anaesthesiology with a deep passion for learning and sharing medical knowledge. Dedicated to advancing the field, Dr. Lima strives to bring fresh, evidence-based insights to the medical community. Viewing medicine not merely as a profession but as a lifelong journey of continuous learning, Dr. Lima is committed to making complex information clear, practical, and useful for healthcare professionals around the world.
Seasonal changes have long been suspected to influence mood and psychiatric disorders. Clinical observations confirm that symptoms fluctuate throughout the year, yet the underlying mechanisms remain unclear. A recent review published in the Journal of Clinical Medicine (2025) by Modzelewski and colleagues introduces the immunoseasonal theory, offering an insightful immunological perspective on how weather patterns impact mental health.
Seasonal Patterns in Mental Disorders
Depression tends to peak during autumn and winter, a period marked by low sunlight, cold temperatures and high humidity. These conditions coincide with increased antidepressant use and a higher risk of postpartum depression in late-year births. Anxiety disorders and post-traumatic stress disorder share similar inflammatory pathways making them equally vulnerable to winter exacerbations.
Schizophrenia shows a different seasonal pattern: hospitalizations frequently rise in summer, with some research noting a secondary peak in winter. Factors such as light exposure and melatonin regulation may play a role in modulating immune responses in these patients.
Bipolar disorder also demonstrates clear seasonality, with manic episodes surging in summer and early autumn, while depressive episodes cluster in the colder months.
The immunoseasonal theory explained
The immunoseasonal theory proposes that seasonal fluctuations in psychiatric symptoms are driven by shifts in immune system activity, particularly the balance between T-helper 1 (Th1) and T-helper 2 (Th2) responses.
- Th1 response (winter): pro-inflammatory state characterized by elevated cytokines like IL-1β, IL-6, TNF-α and IFN-γ. This inflammatory state may impair prefrontal cortex (PFC) function, reducing its regulatory control over limbic structures such as the amygdala and hippocampus. Clinically, this manifests as increased rumination, heightened anxiety and depressive symptoms. The Th1-driven state also interacts with the hypothalamic–pituitary–adrenal (HPA) axis, promoting cortisol dysregulation and circadian rhythm disturbances, which further exacerbate mood disorders.
- Th2 response (summer): anti-inflammatory profile, linked to cytokines such as IL-4, IL-10, IL-13 and TGF-β. While anti-inflammatory in nature, Th2 activation can paradoxically destabilize neural circuits. By stimulating cortical and mesolimbic pathways, Th2 responses may contribute to psychotic symptoms, manic episodes, impulsivity and increased suicide risk. This pattern aligns with epidemiological data showing peaks in schizophrenia exacerbations and mania during warmer months, often coinciding with high allergen exposure and prolonged sunlight.
These immune shifts do not act in isolation. They interact with neurotransmitters such as serotonin and dopamine, disrupt circadian rhythms and amplify stress pathways through HPA axis dysfunction. They also alter brain network connectivity, creating seasonal vulnerability: winter favors depression and anxiety, while summer increases the risk of psychosis, mania, and impulsive behaviors.
Environmental and weather influence
Weather strongly influences immune balance and psychiatric risk.
In winter, cold temperatures and high humidity promote viral infections, amplifying Th1-driven inflammation and worsening depression and anxiety.
Summer heat and allergens shift immunity toward Th2 dominance, increasing vulnerability to psychosis, mania, and impulsive behaviors. Heatwaves and extreme temperatures correlate with higher hospitalizations for schizophrenia and mood disorders. Sunlight generally improves mood but can temporarily raise suicide risk in spring.
Other factors, such as low atmospheric pressure and strong winds, show modest links to impulsivity and psychiatric admissions, though evidence remains inconsistent.
Clinical implications and future directions
If this theory holds true, it could change how we approach mental health care. Care could become more personalized based on the season by adjusting medication doses, adding anti-inflammatory or anti-allergic strategies and using biomarkers like IL-6 or Th1/Th2 ratios to predict risk.
Still, much of the evidence is observational. We need long-term studies, standardized methods, and tools like neuroimaging and circadian rhythm assessments to confirm these links. Understanding how seasons shape our minds could help us move from reactive care to proactive prevention—making psychiatry smarter, more precise, and truly patient-centered.
Read next: Winter depression: can vitamin d shed light on mood?
About the Author – Carolina Lima
Anaesthesiology specialist
Carolina is a specialist in Anaesthesiology with a deep passion for learning and sharing medical knowledge. Dedicated to advancing the field, Dr. Lima strives to bring fresh, evidence-based insights to the medical community. Viewing medicine not merely as a profession but as a lifelong journey of continuous learning, Dr. Lima is committed to making complex information clear, practical, and useful for healthcare professionals around the world.
Last press reviews
Seasonality and mental health: how winter and summer shape our minds
By Carolina Lima | Published on January 6, 2026 | 3 min read
Winter depression: can vitamin d shed light on mood?
By Ana Espino | Published on January 5, 2026 | 3 min read
Holiday weight gain: small indulgences, big effects?
By Ana Espino | Published on January 2, 2026 | 3 min read<br>