Contrast

Metrics

Every number, and how much to trust it

Contrast documents 14 metrics. Each one carries how it is measured, what moves it that has nothing to do with the thing being measured, and one of four labels for how strong the evidence behind it is. 40 citations ship inside the app.

Established

4 of 14

Measured the same way in published research, with results you can compare against.

Borrowed

5 of 14

Based on research into a closely related measure, applied here to sauna and cold.

Theoretical

2 of 14

Contrast’s own measure. It rests on known physiology but hasn’t been tested in a study, so read it as a personal trend.

Your data

3 of 14

A description of your own sessions and Apple Health data. It shows what goes together for you, not what causes what.

Two of those labels say the measure has not been tested in a study, and three say the figure describes your own data rather than anything published. That is a third of the Analysis tab declining to claim backing it does not have.

  1. Weekly goal

    Borrowed

    Weekly minutes of heat and of cold to aim for. The presets follow published routines; they're starting points, not prescriptions.

    6 sources

  2. Sessions and your nights

    Your data

    How your sleep, HRV, breathing rate, wrist temperature and next-morning resting heart rate on nights after a session compare with your other nights.

    5 sources

  3. When you do it

    Your data

    When your sessions start, and what comes before them: waking up, a workout, or bed.

    No external sources — it describes your own data

  4. Over the months

    Your data

    Your weekly sessions lined up with weekly HRV, resting heart rate and VO2 max from Apple Health.

    No external sources — it describes your own data

  5. Sauna sessions

    Established

    How often and how long you sit in the sauna, set against the groups in the largest long-term sauna studies.

    3 sources

  6. Hot baths

    Established

    How often you take a hot bath, set against the groups in a large Japanese study of tub bathing.

    2 sources

  7. Weekly cold and heat

    Borrowed

    Your weekly minutes of cold and of heat, next to the routine of a group of experienced winter swimmers, and your cold minutes by water temperature.

    3 sources

  8. Heart work in the heat

    Borrowed

    Minutes each week your heart worked at an exercise-like level while you were in the heat.

    3 sources

  9. Cold settle

    Borrowed

    How quickly your heart calms after you get into cold water. With practice the first surge passes sooner.

    3 sources

  10. Cold shock

    Established

    The jump in heart rate as you get into cold water from rest. It gets smaller as your body gets used to cold.

    3 sources

  11. Heat strain

    Borrowed

    How hard your heart works at the same heat: your heart rate above resting, five minutes into your first heat round. It falls as you get used to heat.

    4 sources

  12. Heat adaptation

    Theoretical

    An estimate of how adapted to heat you are, built from your recent days of sauna and hot bath, and checked against your heat strain.

    3 sources

  13. Heart rate by water temperature

    Theoretical

    Every hot bath and plunge on one chart: your settled heart rate against the water's temperature. Recent weeks are solid, earlier ones faded, so you can see the curve move.

    2 sources

  14. Resting heart rate

    Established

    Apple Watch's daily resting heart rate: your 28-day median and how it has moved, and whether it differs on days after a session.

    3 sources