Contrast

Cold settle

BorrowedBased on research into a closely related measure, applied here to sauna and cold.
What it is
How quickly your heart calms after you get into cold water. With practice the first surge passes sooner.
How it is measured
For each plunge of 3 minutes or more: the seconds from the highest 15-second heart rate in the first 45 seconds until heart rate has come halfway down to where it sits between minutes 2 and 3. Plunges are compared only within one water band (below 10 °C, 10–15, 15–20, 20 °C or warmer), the band you've used most lately. Your first 3 plunges are set against your latest 3. It works whether you plunge from rest or straight after heat.
How to read it
Lower is quicker. In lab studies the early heart and breathing response to cold water got shorter with repeated immersions, even when its size didn't change, and heart rate over the first five minutes fell from about 90 to 78 bpm. Your own trend is what matters here; there's no population range.
What it cannot tell you
Contrast's own measure, built on the lab findings rather than tested itself. How fast you get in, how deep, breathing, and moving about all change it. Wrist heart rate trails a sudden change by several seconds and can drop readings in cold water.

Sources

Every one of these ships inside the app, with the finding written out so you do not have to open the paper to know why it is there.

  1. Tipton MJ, Mekjavic IB, Eglin CM. Permanence of the habituation of the initial responses to cold-water immersion in humans. European Journal of Applied Physiology, 2000.

    After habituation the initial responses to cold water were shorter as well as smaller, and the change lasted 7 to 14 months.

    doi.org/10.1007/s004210000255

  2. Eglin CM, Costello JT, Bailey SJ, Gilmour S, Tipton MJ. Effects of repeated cold-water immersion on the cold shock response. Extreme Physiology & Medicine (abstract), 2015.

    After repeated immersions, heart rate between 60 and 300 seconds fell from about 90 to 78 bpm.

    doi.org/10.1186/2046-7648-4-S1-A38

  3. Tipton MJ, Stubbs DA, Elliott DH. Human initial responses to immersion in cold water at three temperatures and after repeated immersions. Journal of Applied Physiology, 1991.

    Colder water made the initial response last longer, which is why Contrast only compares plunges at similar temperatures.

    doi.org/10.1152/jappl.1991.70.1.317