The Science
Tidal Contrast is built on published research, and we say how strong each finding is. Most cold and heat research is small, short or observational, so treat everything here as a reason to be careful, not a promise of results.
Last Reviewed 27 October 2026
Sudden cold water triggers a gasp, fast breathing and a spike in heart rate and blood pressure in the first minute or two. Work by Mike Tipton and colleagues shows this response fades with repeated, short, safe exposures. That is why the app starts with short limits and a 90 second settle.
Your body can keep cooling for a while after you leave cold water, as cold blood from the skin and limbs returns to the core. This is the reason every session ends with a rewarm, and why the app asks you to wait before a hot shower.
Studies of regular winter swimmers, including work by Susanna Søberg and colleagues, report changes in how the body generates heat. The samples are small, so we treat the results as interesting, not settled, and the app makes no promises about them.
Large observational studies in Finland by Jari Laukkanen and colleagues linked frequent sauna use with lower cardiovascular risk. Observational means the sauna may not be the cause, so the app makes no health promises about it.
Sitting in heat raises heart rate in a way that looks like light exercise. That is why heart rate on the wrist matters, and why every sauna round has a clear point to step out.
Figures of about 11 minutes of cold and 57 minutes of heat a week come from Susanna Søberg's reading of her own research. They are a reference point, not a medical guideline, and the app never pushes you to reach them.
Tidal Contrast will not show a number the Watch cannot measure. Depth is used only to notice when you go in and come out, not to measure how deep you are. Other Watches run a full session without live water temperature.
