Does Winter Swimming Change Your Stress Hormones Over Time?
Yes, but so does simply being in a research study. This Finnish follow-up study tracked winter swimmers and non-swimmer controls across one winter swimming season. Both groups showed decreasing catecholamines (noradrenaline and adrenaline) from autumn to spring. The winter swimmers’ systolic blood pressure also fell significantly.
Researchers from the University of Oulu followed winter swimmers and non-swimmer controls over one winter swimming season. They measured blood hormones and their metabolites on three occasions, in autumn, winter and spring, and compared those results to psychological traits recorded at each blood sampling. 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 across a full winter season, the changes showed up in both swimmers and non-swimmers. The declines in noradrenaline and adrenaline might reflect cold adaptation in the swimmers, but they might also reflect adaptation to the research situation itself, or ordinary seasonal variation from autumn to spring. I appreciate the authors’ honesty: “No clear effect of winter swimming as such was detected.”
Study Design
Participants:
- A group of winter swimmers
- A group of non-swimmer controls
Measurements: Blood samples collected on three occasions, in autumn, winter and spring, to measure:
- Noradrenaline and adrenaline (catecholamines)
- Serotonin and its metabolite 5-HIAA
- Homovanillic acid, or HVA (a dopamine metabolite)
- Beta-endorphin
Blood pressure was recorded at the same time points.
Psychological testing: Psychological traits were recorded at the time of each of the three blood samplings, including anxiety, hysteria and obsessionality.
Key Findings
Blood pressure: Mean systolic blood pressure in the winter swimmers fell from 134 +/- 12 mmHg to 128 +/- 12 mmHg (p < 0.05) during the winter. Controls showed a slight drop as well, but it was not statistically significant.
Noradrenaline: Mean plasma concentrations diminished significantly from autumn to spring, and more so in the winter swimmers. Even so, no statistically significant difference was observed between the two groups.
Adrenaline: Levels showed a decreasing trend, and the change reached significance when calculated using the combined means of both groups.
Serotonin: Plasma levels decreased in both groups by about 50 per cent by spring, while 5-HIAA did not change significantly.
Beta-endorphin and HVA: Plasma values were at the same level across all seasonal samples in both groups.
The Adaptation Question
The authors read the changes in humoral status as speaking for adaptation to the research situation, or as reflecting seasonal variation from autumn to spring.
Since both groups showed similar changes, winter swimming did not produce hormonal effects beyond what the controls experienced.
Some Correlations Found
The researchers found a few connections between hormones and psychological traits:
- In autumn, HVA correlated with blood pressure and anxiety (r = 0.367)
- In winter, beta-endorphin and hysteria correlated negatively (r = 0.370)
- In spring, 5-HIAA and obsessionality correlated positively (r = 0.351)
These correlations suggest some relationship between hormone levels and psychological traits, but they were modest and scattered across different time points.
Why Swimmers Say They Feel Better
The humoral measurements here were taken on three occasions across a season, not during a dip. That leaves room for explanations this design could not capture:
- Activation of the sympathetic nervous system during the cold immersion itself, even if the seasonal samples do not differ between groups
- Psychological factors such as achievement and community
- Acute effects during swimming that do not show up in periodic blood samples
Practical Takeaways
- Systolic blood pressure in the winter swimmers fell significantly across the season, by roughly 6 mmHg
- Catecholamine and serotonin changes from autumn to spring occurred whether the person swam or not
- The subjective benefits swimmers report may work through mechanisms not captured by seasonal hormone sampling
- On the authors’ own reading, no clear effect of winter swimming as such was detected
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?
Across one winter season, the plasma levels measured in swimmers were not significantly different from those in non-swimmers. Both groups showed decreasing catecholamines from autumn to spring, and both showed plasma serotonin about 50 per cent lower by spring.
Why do winter swimmers report feeling better if hormones aren’t different?
This study sampled blood on three occasions across a season, so it speaks to seasonal levels rather than to the response during a dip. Benefits may come from the acute response to cold immersion, or from psychological factors such as challenge and community, none of which the seasonal samples would show.
Did the study find any unique effects in winter swimmers?
Blood pressure came closest. Mean systolic pressure in the winter swimmers fell from 134 mmHg to 128 mmHg (p < 0.05), while the drop in the controls was slight and not statistically significant. Noradrenaline also fell more in the swimmers, but the difference between groups was not statistically significant.
Bottom Line
This Finnish follow-up study tracked winter swimmers and non-swimmer controls across one winter swimming season, measuring blood pressure, catecholamines, serotonin and its metabolite, HVA, beta-endorphin, and psychological traits on three occasions. Winter swimmers showed a significant drop in systolic blood pressure, from 134 +/- 12 mmHg to 128 +/- 12 mmHg. The hormonal changes were similar between the groups: noradrenaline and adrenaline decreased from autumn to spring, and plasma serotonin was about 50 per cent lower by spring in both groups. The authors concluded that these changes speak for adaptation to the research situation or reflect seasonal variation, and that no clear effect of winter swimming as such was detected.
