Cold shock
- What it is
- The jump in heart rate as you get into cold water from rest. It gets smaller as your body gets used to cold.
- How it is measured
- Your highest 15-second heart rate in the first 45 seconds in the water, minus your median heart rate from 60 to 10 seconds before you got in. The lab studies used the first 30 seconds; the extra 15 cover the watch's delay. Only plunges in water of 20 °C or colder, taken at least 5 minutes after any heat, count, and they're compared within one water band. Your first 3 are set against your latest 3.
- How to read it
- A falling number means the reflex is settling with practice. In lab studies it shrank after four to six immersions, carried over to colder water, and stayed smaller for months, then levels off: steady after that is expected.
- What it cannot tell you
- Plunges straight after a sauna start with heart rate already high and can't show it. Anxiety, how fast you get in and how deep all change it. Cold water can make wrist readings patchy, so a single plunge can be off.
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.
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 six 3-minute immersions at 15 °C, heart rate in the first 30 seconds at 10 °C fell from 128 to 109 bpm, and was still lower 7 to 14 months later.
doi.org/10.1007/s004210000255Barwood MJ, Corbett J, Tipton MJ, et al. Habituation of the cold shock response is inhibited by repeated anxiety. Frontiers in Psychology, 2018.
Anxiety about the immersion made the heart rate response larger and slowed its habituation.
doi.org/10.3389/fpsyg.2018.00510Tipton MJ, Collier N, Massey H, Corbett J, Harper M. Cold water immersion: kill or cure? Experimental Physiology, 2017.
How fast the skin cools drives the response, so speed and depth of entry matter.
doi.org/10.1113/EP086283