Does Winter Swimming Change Your Stress Hormones Over Time?
Yes, but so does simply being in a research study. This Finnish study tracked winter swimmers and non-swimmers over one winter season. Both groups showed decreased stress hormones (noradrenaline and adrenaline) from autumn to spring. The winter swimmers’ blood pressure also dropped significantly.
Researchers from the University of Oulu followed 25 winter swimmers and 11 non-swimmer controls over one winter. They measured blood hormones and psychological traits three times: October, January, and May. The goal was to understand whether winter swimming produces lasting changes in stress hormones and brain chemicals.
Dr. Kumar’s Take
This study is a reality check. Winter swimmers claim the practice reduces stress and improves mood. But when researchers actually measured stress hormones over a full winter season, they found the changes happened in both swimmers and non-swimmers. The drops in noradrenaline and adrenaline might reflect cold adaptation in swimmers, but they might also just be adaptation to the research study itself. I appreciate the authors’ honesty: “No clear effect of winter swimming as such was detected.”
Study Design
Participants:
- 25 winter swimmers (18 women, 7 men, average age 55)
- 11 non-swimmer controls (8 women, 3 men, average age 54)
Measurements: Blood samples collected in October, January, and May to measure:
- Noradrenaline and adrenaline (stress hormones)
- Serotonin and its metabolite 5-HIAA
- Dopamine metabolite (HVA)
- Beta-endorphin
Psychological testing: Questionnaires measuring anxiety, depression, phobias, and other psychological traits
The swimmers took a cold water dip an average of 5-6 times per week.
Key Findings
Blood pressure: Systolic blood pressure in winter swimmers dropped from 134 mmHg to 128.5 mmHg (p < 0.05) over the winter. Controls also dropped (from 131 to 124) but this wasn’t statistically significant.
Noradrenaline: Dropped significantly in both groups from autumn to spring (p < 0.001). The swimmers dropped 23.3% and controls dropped 17.7%.
Adrenaline: Also dropped in both groups (p < 0.05). Swimmers dropped 43% and controls dropped 32%.
Serotonin: Decreased by about 50% from autumn to spring in both groups (p < 0.001).
Beta-endorphin: Remained stable throughout the winter in both groups.
The Adaptation Question
The researchers couldn’t tell whether the hormone changes were due to:
- Cold adaptation in the swimmers
- Mental adaptation to the research study itself
- Normal seasonal variation from autumn to spring
- All of the above
Since both groups showed similar changes, winter swimming didn’t produce unique hormonal effects beyond what controls experienced.
Some Correlations Found
The researchers found a few connections between hormones and psychological traits:
- In autumn, HVA (dopamine metabolite) correlated with anxiety
- In winter, beta-endorphin negatively correlated with hysteria
- In spring, 5-HIAA correlated with obsessionality and blood pressure
These correlations suggest some relationship between hormone levels and mood, but the connections were weak and scattered across different time points.
Why Swimmers Say They Feel Better
Despite no clear hormonal differences, winter swimmers consistently report feeling refreshed and energized. The researchers suggest the beneficial effects might work through:
- Activation of the sympathetic nervous system during the dip itself (even if baseline levels don’t differ)
- Changes in thyroid hormones (not measured in this study)
- Psychological factors like achievement and community
- Acute effects during swimming that don’t show up in resting hormone levels
Practical Takeaways
- Winter swimming may lower blood pressure over a season
- Stress hormone changes from autumn to spring occur whether you swim or not
- The subjective benefits swimmers report may work through mechanisms not captured by resting hormone levels
- Both swimmers and non-swimmers in this study were psychologically healthy
Related Studies and Research
- Related Podcast Episode
- RCT: cold-water immersion vs massage for DOMS after CrossFit Murph
- SYSTEMATIC REVIEWS, EVIDENCE MAPS, AND BIG-PICTURE OVERVIEWS
- Effects of cold stimulation on cardiac-vagal activation and HRV
- Cardiovascular and mood responses to an acute bout of cold-water immersion
FAQs
Does winter swimming change your hormone levels?
Acute cold water exposure definitely increases stress hormones temporarily. But when measured over a winter season, the resting levels in swimmers weren’t different from non-swimmers. Both groups showed decreased hormones from autumn to spring.
Why do winter swimmers report feeling better if hormones aren’t different?
The benefits may come from the acute response during swimming rather than lasting hormonal changes. There may also be psychological benefits from challenge, community, and connection with nature that don’t show up in hormone tests.
Did the study find any unique effects in winter swimmers?
The only significant difference was in blood pressure. Winter swimmers had a significant drop in systolic blood pressure (6 mmHg) over the winter, while the drop in controls wasn’t statistically significant.
Bottom Line
This Finnish study tracked 25 winter swimmers and 11 controls over one winter season, measuring stress hormones, brain chemicals, and psychological traits. While winter swimmers showed a significant drop in blood pressure, the hormonal changes were similar between swimmers and non-swimmers. Noradrenaline, adrenaline, and serotonin all decreased from autumn to spring in both groups. The researchers concluded that these changes might reflect adaptation to the research study or seasonal variation rather than specific effects of winter swimming. The subjective benefits that swimmers report may work through mechanisms not captured by resting hormone measurements.
