Then and now: your first three rounds against your latest three

Three adaptation measures, one comparison shape. Cold shock, cold settle and heat strain all take your first three qualifying rounds and set them against your latest three, and the chart draws both as curves rather than two bars.

Why three

Three is a compromise and both sides of it are worth stating.

Against more: qualifying rounds are scarcer than they look. Cold shock needs a plunge at 20 °C or colder, at least five minutes clear of any heat, in the same water band as the others. Someone doing a conventional sauna-plunge-sauna-plunge session produces none of those. Someone plunging standalone twice a week in an outdoor barrel produces maybe six a month, spread across two bands as the water warms.

A comparison requiring ten a side would be empty for most people for most of a year, and an empty card teaches nobody anything.

Against fewer: one round against one round is a coin flip. Cold water makes optical heart rate patchy — vasoconstriction moves blood away from where the sensor is looking — so a single plunge’s trace can be thin exactly where the peak is.

Three is the smallest number that survives one bad reading.

Why curves rather than averages

Two averages would report a difference and hide everything about it. The chart draws the heart rate curve for each of the six rounds, early ones in one treatment and recent ones in another, so you can see the shape.

That distinction does real work at n=3. A genuine adaptation looks like three recent curves sitting below three early ones. A misleading average looks like two recent curves overlapping the early ones and one sitting far below — which is one unusually calm plunge, not a trend.

At three rounds a side, that difference is common enough that the chart earns its place over a number.

The banding, and why it empties charts

Every one of these comparisons happens within a single temperature band, using the band you have used most recently.

For cold, the bands are below 10 °C, 10–15, 15–20, and 20 °C or warmer. For heat, baths match within 1 °C of measured water and saunas within 10 °C of the air temperature you typed in.

The reason is measured rather than theoretical. Colder water makes the initial response last longer as well as hit harder, so a trend mixing 3 °C and 15 °C plunges would mostly be a chart of the season. The 1991 study that established this compared responses at three temperatures directly.

The cost is that an outdoor barrel drifting from 4 °C in February to 16 °C in June crosses three bands, and each crossing starts a new comparison group. This is the single most common reason a card is empty after what feels like plenty of plunges.

What the comparison can and cannot establish

It is a before-and-after with no control, which is the weakest useful design there is.

What it can show: that your response at the same water temperature, measured the same way, is different now than when you started.

What it cannot separate: adaptation from technique, from fitness, from the season, from a better-fitting watch strap. Someone who learned to breathe through the entry between round three and round twenty has improved genuinely and not physiologically.

The research gives a shape to expect, which helps. Habituation of the cold shock response arrives in about four to six immersions and then plateaus, and it persists for 7 to 14 months. Heat acclimation arrives mostly in the first week and decays at about 2.3% a day without heat. A trend that matches those timings is more likely to be the thing they describe. A steady improvement continuing linearly over a year is more likely to be something else.

The awkward consequence of “first three”

The early group never updates. Your first three qualifying plunges stay your first three forever, which means the comparison is always against whoever you were then.

That is right for the question have I adapted since I started and wrong for the question am I still adapting. The second question is better answered by watching the recent curves move over successive months, which the faded-versus-solid treatment on the heart rate by water temperature scatter is better at showing.

Two charts, two questions. Neither is the whole answer.

When to look

Not early. With five qualifying plunges, your first three and your latest three share one round, and the comparison is largely a plunge against itself.

Ten in one band is where it starts to mean something. For most people that is two or three months, and for someone who only plunges after a sauna the cold shock card will never populate at all — which is a fact about the measurement rather than about them.