Sauna, Ice Baths and Resting Heart Rate: Logging Sessions for Cleaner Trends

Saunas and cold plunges both change your heart rate quickly, and both usually involve sitting still, which could look like rest to a wearable. This guide covers what the research does and doesn't show, some safety basics, and how logging sessions with Heat & Cold in My Bio Age helps keep them from blurring the resting heart rate My Bio Age uses.

What heat and cold do to your heart rate

A hot sauna makes your heart work harder, not less. In a small 2019 study, 19 healthy adults spent 25 minutes in a sauna at 93°C. Their heart rate and blood pressure rose steadily, and the researchers likened the load on the heart to moderate exercise. Both fell during the 30-minute rest afterwards.

Longer-term claims are less settled. A 2018 review by Laukkanen and colleagues gathered research on Finnish sauna bathing, including links with lower risk of high blood pressure and cardiovascular disease. The authors call this emerging evidence and the mechanisms proposed rather than proven. Associations like these cannot show that the sauna itself made the difference.

Cold water acts differently. Sudden immersion triggers cold shock, which the US National Weather Service describes as a gasp with rapid changes in breathing, heart rate and blood pressure. A 2022 review by Esperland and colleagues notes that cooling the face while holding your breath can instead slow the heart. Most of the 104 studies it included were small, so whether regular cold exposure benefits health is still debated.

For tracking, the point is simpler: during and soon after a session, your heart rate is not your resting heart rate.

Safety basics

• Start short and build up gradually.
• Drink water and skip alcohol. NHS advice on preventing heat exhaustion and heatstroke includes drinking more and avoiding too much alcohol.
• Get out if you feel unwell. The NHS lists dizziness, headache, nausea, cramps and intense thirst among the signs of heat exhaustion. Cool down and drink fluids, and call emergency services if you are still unwell after 30 minutes, or have signs of heatstroke such as a fast heartbeat or confusion.
• Ask a clinician first if you have a heart condition, high or low blood pressure, or are pregnant. HealthLink BC advises against overheating while pregnant, since a rise in core temperature may harm the baby, with the first weeks the most sensitive time.
• Leave Apple Watch outside the sauna. Apple's handling guide says to avoid wearing Apple Watch in a sauna or steam room (for Ultra models, a sauna above 55°C), as it may affect water resistance. Start the timer on iPhone before you go in, and put the watch back on afterwards.
• Never swim in cold water alone. The National Weather Service warns that the gasp and fast breathing of cold shock can make drowning more likely, however well you swim.

Why logging matters

Apple's HealthKit documentation describes resting heart rate as an estimate of your lowest heart rate during periods of rest, worked out from sedentary readings across the day. Sitting in a sauna or ice bath, or resting quietly afterwards, could look like one of those periods, even though heat or cold is shaping your heart rate. Apple notes that a sedentary reading isn't automatically a resting one, but doesn't say how its estimate treats time in a sauna or plunge. So Heat & Cold marks those windows, and My Bio Age sets them aside as a precaution.

How Heat & Cold works in My Bio Age

On iPhone, the dashboard's Heat & Cold card reads “Log a sauna or ice bath so it does not count as resting”. Tap it, choose Start Sauna or Start Ice Bath, and tap Finish when you are done. The app's Start Sauna, Start Ice Bath and Finish Session shortcuts do the same, opening My Bio Age to run the timer.

With your permission, each finished session within the time limits below is saved to Apple Health as a Preparation and Recovery workout, because HealthKit has no sauna or cold-water workout type.

On days with a logged session, My Bio Age then works out resting heart rate itself:

1. It reads that day's raw heart rate from Apple Health and leaves out the session plus a settling period: 30 minutes after a sauna, 15 after an ice bath.
2. From the quietest remaining readings, it estimates where your heart rate settled.
3. It aligns that estimate with Apple's scale by comparing the two methods on recent session-free days. It needs at least five comparable days.
4. Without them, or with too few readings left that day, it keeps Apple's value and marks it internally as uncorrected.

The weekly MyBioAge Index™, Recovery (Recovery Capacity) and Stress Resilience then use the corrected value for that day. Your resting heart rate in Apple Health is left unchanged.

Current limits

• Log sessions on iPhone. My Bio Age on Apple Watch has Heat & Cold too, but the iPhone app doesn't read watch sessions back, so they don't feed the correction yet.
• Wear your watch for the rest of the day. The correction works from heart-rate readings outside the session, which normally come from Apple Watch, and only on days that also have a resting heart rate from Apple.
• Only My Bio Age sessions count. A sauna recorded in another app isn't recognised.
• Stop the timer. A sauna running past three hours, or an ice bath past one, is treated as a forgotten timer: it isn't saved to Apple Health or used for the correction. A forgotten timer under those limits still sets aside more of your day than it should.

Reading your trends

• Compare weeks, not single days. The resting heart rate detail view charts weekly averages with the week-over-week change.
• Check your sessions were logged. The app doesn't show which days were corrected. If a week with sessions still looks out of line, check the Recent list on the Heat & Cold screen for each session and its length. A change can also come from sleep, travel or illness.
• Don't chase a lower number. Heat & Cold is about a cleaner measurement, not a better-looking score.

Our resting heart rate guide explains how to read a change, and the Recovery Capacity explainer and Stress Resilience guide show how each score uses it. If sessions are part of your training week, see how to balance training load and recovery.

Putting it together

Whether you use a sauna or a cold plunge is your choice. My Bio Age's job is to keep those sessions out of the resting heart rate it uses: log them on iPhone, finish the timer when you get out, and read resting heart rate across weeks.

If heat or cold sessions are already part of your routine, download My Bio Age free on the App Store to log them on iPhone and keep their timing with your health trends. The iPhone app is completely free, with no in-app purchases, subscription or account required. Session records and resting heart rate calculations stay private on your device.

My Bio Age provides general wellness information, not medical advice. It cannot tell you whether heat or cold is safe for you. If you feel faint, dizzy or unwell, or notice a racing or irregular heartbeat, get out straight away, then cool down or warm up. The NHS says to call 999, the UK emergency number, for chest pain that comes on suddenly and doesn't go away, or that comes with sweating, sickness, light-headedness or shortness of breath. Elsewhere, call your local emergency number. Talk to a clinician about any symptoms before your next session.