The impact of stress on health is multifaceted, with acute stress sometimes leading to advantageous short-term effects while chronic stress causes detrimental long-term consequences. Stress-induced analgesia (SIA), the reduction in pain perception due to stress, is a phenomenon previously studied primarily in laboratory settings. While anecdotal evidence from World War II suggested SIA's existence in real-world high-stress situations, systematic ecological research has been limited due to the difficulty of measuring stress and pain responses during such events. The ongoing Russia-Ukraine conflict presents a unique opportunity for such research, given its intensity and scale. This study leverages data from Flo, a widely used women's health app, to investigate the relationship between self-reported stress and pain among Ukrainian women during the initial stages of the conflict. The researchers hypothesized an inverse relationship between stress and pain, consistent with SIA, anticipating this effect to diminish over time as Ukrainians adapted to the ongoing war. They also hypothesized that this effect, while present in neighboring countries, would be less pronounced than in Ukraine.

Existing literature extensively documents the influence of stress on cognitive, perceptual, and sensory processing. Acute stress can improve memory for stressor-relevant information, yet it also affects pain perception, leading to SIA. Laboratory studies consistently demonstrate pain suppression following exposure to stressors, revealing underlying anatomical, neurochemical, and molecular mechanisms. However, most studies have been conducted in controlled lab settings. Research on the health consequences of armed conflict demonstrates devastating effects on physical and mental health, with women often exhibiting heightened vulnerability to stress due to increased societal and familial responsibilities during conflict. Prior research on conflict zones like Sri Lanka, the Balkans, and Afghanistan shows high rates of psychiatric disorders among civilians, particularly women.

This study used data from the Flo app, a women's health and wellness app with over 57 million monthly active users worldwide. The researchers focused on a cohort of Ukrainian users who actively logged multiple symptoms before and after the start of the Russia-Ukraine conflict (February 24, 2022). To mitigate selection and collider bias, the analysis was restricted to 87,315 users who consistently logged symptoms before and after the war's start. Descriptive statistics were obtained for symptom prevalence before and after the conflict's onset. A logit model was used to examine the relationship between self-reported stress and pain, with pain as the dependent variable and stress and other symptoms as independent variables. The model controlled for time fixed effects and the interaction between stress and time to account for temporal dynamics. The marginal effect of stress on pain was examined across time. To assess the specificity of the stress-pain relationship, alternative regressions were conducted, replacing stress with other symptoms as independent variables. The geographic extent of the effect was explored by analyzing data from 38 European countries.

The study revealed a significant inverse relationship between logged stress and pain reports among Ukrainian Flo users following the war's commencement. The prevalence of logged stress surged dramatically on February 24, 2022, while the prevalence of various pain-related symptoms concurrently decreased. The marginal effect of stress on pain was substantial immediately following the war's start (-0.572), considerably stronger than the pre-war effect (-0.161). This significant reduction in the likelihood of reporting pain in response to stress persisted for approximately 15 days. Analysis of alternative regressions showed that the impact of stress on pain was specific and more profound than its effect on other logged physiological or mood symptoms, with mood swings and feelings of calmness being the other two significantly affected. Geographical analysis indicated that the largest effect size was observed in Ukraine, with a significant, though smaller effect also found in neighboring countries like Poland, Czechia, and Moldova.

The findings provide real-world evidence of stress-induced analgesia (SIA) during a period of extreme stress, complementing previous lab-based studies. The observed inverse relationship between stress and pain suggests an acute, adaptive mechanism in the face of a catastrophic event. The fact that the effect lessened over time suggests it was acute rather than a persistent phenomenon. However, while SIA offers a short-term adaptive advantage, the long-term health consequences of chronic stress and trauma are severe, including increased risk of chronic pain and PTSD. This underscores the importance of providing adequate mental health support and addressing trauma-related issues in conflict-affected populations. The study's use of a large, readily available dataset from a mobile health app demonstrates the potential of digital technologies for ecological research on stress and health outcomes in challenging circumstances.

This study utilizes data from the Flo app to demonstrate a real-world example of stress-induced analgesia (SIA) during the Russia-Ukraine conflict. The observed inverse relationship between stress and pain highlights the complex interplay between acute stress and pain perception. While offering a potential short-term adaptive response, the long-term health implications of sustained stress necessitate attention to mental health support for affected populations. Future research incorporating physiological measures like cardiovascular data could further elucidate the underlying mechanisms of SIA.

The study relies on self-reported symptom data, which may be subject to bias due to variable reporting patterns and potential underreporting of symptoms. The analysis focused on users active before and after the war's onset, potentially excluding users who ceased app usage due to the conflict. The study did not track location changes dynamically throughout the conflict. While efforts were made to minimize selection and collider bias, the potential for residual bias remains. The study also used location settings at the time of the last login rather than dynamic location data to determine geographical effects.