Logging a sauna session without an Apple Watch

The automatic detection needs an Apple Watch Ultra or a Series 10 and later. Everything else in Contrast needs an iPhone, and a good deal of the app works with no watch at all.

Two ways to log by hand

Live. Start a session on the iPhone and it runs a timer, with the same Switch control the watch has. Put the phone down outside the sauna, come out, switch to cold, go back. The phone is doing what the watch would do minus the sensor readings.

After the fact. Open the log screen and describe a session that already happened: when it started, then each round in order with its modality and its length. Add temperatures if you know them, skip them if you do not.

The second is the one most people use, because the bag with the phone in it is rarely within reach. It takes about twenty seconds for a three-round session.

What a manual session needs

The start time and the rounds. Everything else is optional.

A round is a modality, a phase and a duration. Rest between rounds is a round too — it has its own phase and it is counted, so a session logged as sauna 15, rest 5, plunge 2, rest 5, sauna 12 records 27 minutes of heat, 2 of cold and 10 of rest, and the day ring draws all three.

Temperatures are per round. Type the water temperature for a bath or plunge and the round joins the band chart; leave it blank and the minutes still count towards your weekly cold or heat. For a sauna you can type the air temperature, which is worth doing if you use more than one sauna — the heat strain metric compares saunas within 10 °C of each other and groups them all together when it has no reading.

What a manual session can and cannot feed

This is the honest part, and it splits cleanly along one line: does the metric need heart rate?

The pattern is worth stating outright: the habit and timing analysis works fully by hand, and the adaptation analysis does not. Cold shock is the size of the jump in your heart rate when you get in; with no heart rate there is no jump to measure. Nothing is hidden or greyed out to sell you a watch — those cards simply have nothing to draw and say so.

The nights section is the pleasant surprise. It compares your sleep, HRV and next-morning resting heart rate on nights after a session against your other nights, and all of that data comes from Apple Health rather than from the session. What the session supplies is the date. A manual log is a perfectly good date.

The cohort comparison works by hand

The two studies the app sets your habit against both measured self-reported frequency, which is exactly what a manual log is.

The Finnish cohort followed 2,315 men for a median of 20.7 years and grouped them by sessions a week and minutes a session. Against once a week, four to seven sessions went with a lower risk of sudden cardiac death, with a hazard ratio of 0.37 in those men; sessions over 19 minutes went with lower risk than sessions under 11. The Japanese tub-bathing study followed about 30,000 adults for around 20 years and found almost-daily bathing associated with a 28% lower risk of cardiovascular disease than zero to two times a week.

Both are observational, both asked about habits once at the start, and neither shows that sauna or bathing caused the difference — people who bathe more may differ in other ways. Contrast reports which group your last four weeks put you in and repeats those caveats on the metric’s own page.

Your minutes are your minutes whether a sensor counted them or you did.

Free, and staying that way

Manual logging is in the free tier, along with watch tracking, saving to Apple Health, your last seven days of history, your weekly heat and cold set against the research, and the full explanation of every metric.

Contrast Pro adds your full history, the heart rate and water temperature charts, time in each band, heart rate zones, sauna temperature, combining and archiving sessions, and the Analysis tab beyond the weekly amounts.

A phone-only user on the free tier gets a complete log, a calendar, Health integration and the research comparison. That is a real product rather than a demonstration of one.

One habit worth forming

Log the session the same evening. The thing that degrades a hand-written log is not accuracy about minutes — nobody needs that to the second — it is the session you meant to enter on Thursday and entered on Sunday as “about twenty minutes, I think”.

The nights analysis is the reason to care. It compares nights following a session day against nights that did not, and a session attributed to the wrong day moves a night from one side of that comparison to the other. Two or three of those will not matter across six months. A habit of them will.