Apple Watch Low Power Mode and Health Tracking: What Stops Working When You Save Battery

In Low Power Mode, Apple Watch stops background blood oxygen and heart rate measurements (also during sleep), collection of heart data for hypertension notifications, and heart rate notifications for irregular rhythm, high and low heart rate. Workouts still record heart rate and pace. Low Power Mode turns off by itself at 80% battery, unless you set a number of days.

Overnight HRV, which is derived from background heart rate readings, will therefore have a gap too. That part is our reading of Apple's list, not a line Apple states.

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What Low Power Mode is and why it exists

Apple Watch manages battery life by reducing background sensor activity and wireless connections when power is limited. Low Power Mode is the intentional, user-activated version of this reduction. It is designed to extend the time you can keep your watch operational when a charge is not available - on a long trip, during a multi-day outdoor activity, or simply when you forgot to charge overnight.

When Low Power Mode is active, a yellow circle icon appears at the top of the watch face. The battery percentage in Control Center and the charging animation also turn yellow as visual indicators.

Low Power Mode can be activated in Control Center (press the side button → tap the battery percentage → turn on Low Power Mode), in Settings → Battery, or set to activate automatically when you start a workout. When manually enabled, you can choose to have it stay on for 1, 2, or 3 days, or leave it on indefinitely until the battery reaches 80% again.

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Health features that stop working

According to Apple's support documentation (updated September 2025), Low Power Mode turns off the following health-related features:

Background blood oxygen measurements, including during sleep. The optical sensors used for SpO2 measurement run periodically when you are still - typically during sleep - to estimate your blood oxygen level. In Low Power Mode, these background measurements stop. You can still initiate a manual SpO2 measurement from the Blood Oxygen app, but the passive overnight sampling that feeds the Health app's history does not occur.

Background heart rate measurements, including during sleep. Background heart rate polling - which also provides the data used to calculate overnight HRV (SDNN) - is suspended. Apple Watch measures HRV using the variation between heartbeats detected during the optical heart rate readings at night; without those readings there is nothing to calculate overnight HRV from (our inference; Apple does not name HRV in its Low Power Mode list).

Collection of heart data for hypertension notifications. Apple Watch Series 9 and later, and Apple Watch Ultra 2 and later (not available on Apple Watch SE), can detect patterns associated with elevated blood pressure using passive daytime optical heart sensor data. This collection is disabled during Low Power Mode, meaning any period in Low Power Mode produces a gap in the data available for this feature.

Heart rate notifications for irregular rhythm, high heart rate, and low heart rate. These real-time alert features require ongoing background heart rate monitoring. In Low Power Mode, they do not trigger.

One consequence not explicitly named in the Low Power Mode documentation but confirmed in the AFib History feature guide: AFib History background rhythm checks are also suspended. If you rely on AFib History for regular weekly estimates, extended use of Low Power Mode will produce gaps in the weekly burden percentage.

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What stays working during workouts

Low Power Mode does not disable the Workout app's active measurements. When you start a workout in the Workout app while Low Power Mode is on, metrics including heart rate and pace continue to be recorded. Battery lasts longer during the workout compared to not using Low Power Mode, with metrics still available.

An additional option - Fewer GPS and Heart Rate Readings - takes power saving further during outdoor walking, running, and hiking workouts. When enabled (Settings → Workout → Fewer GPS and Heart Rate Readings) and Low Power Mode is active, GPS and heart rate are sampled less frequently. The trade-off is that workout alerts are disabled, splits and segments are not recorded, and current pace is not available mid-workout. This option is available on Apple Watch Series 8 and later, SE 2 and later, and all Ultra models.

You can also configure Low Power Mode to activate automatically when you begin any workout (Settings → Workout → Low Power Mode), which preserves more battery for long-duration activities.

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The gap in overnight health data

The most significant health tracking consequence of Low Power Mode is the overnight gap it creates. Apple lists background heart rate and blood oxygen measurements, including during sleep, as off in Low Power Mode. Overnight HRV is calculated from heart rate readings, so a gap there is expected (our reading of Apple's list). Apple does not say whether wrist temperature deviation or sleep staging are affected.

If you put your Apple Watch in Low Power Mode before bed because the battery was low, and it charged back to 80% during the night (turning Low Power Mode off automatically), the Health app will show a gap in overnight data up to the point when Low Power Mode deactivated.

This is particularly relevant for people tracking HRV trends over time, where a missing night can interrupt a streak or skew a weekly average. The Health app does not label gaps as Low Power Mode periods - a missing night simply appears as absent data.

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Features affected but not disabled

Beyond health monitoring, Low Power Mode also affects other capabilities without fully disabling them: phone calls may take longer to connect, background app refresh slows, complications on the watch face update less frequently, Live Activities update less often, and Siri takes longer to respond. Some scrolling and animations may appear less smooth.

When iPhone is not nearby, Low Power Mode also suspends Wi-Fi and cellular connections. Missed calls and notifications are retrieved approximately once per hour rather than in real time.

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Look at your overnight values in Sam and take them to your appointment

Sam puts the overnight values your Apple Watch records into one app: sleep and sleep stages, resting heart rate, HRV, respiratory rate, blood oxygen and wrist temperature, over 7 days, 30 days or six months.

• Instead of one Health app screen per value: the night and its values can be read next to each other.
• Instead of memory at the appointment: the symptom diary keeps how you felt on a given day, in a few taps.
• Instead of screenshots of single nights: the monthly report you create has a sleep page for the month, plus activity, as a PDF.

Sam can only show what Apple Watch records, so a night without data in Apple Health stays without data in Sam.

Try Sam for free

Sam is a wellness companion, not a medical device. The app does not diagnose, treat, cure or prevent any condition or health state. Always consult qualified professionals for medical decisions.

Related reading

• Apple Watch blood oxygen explained
• sleep stages vs polysomnography
• what AFib History tracks and what it misses

Sources

Updated: October 2026

• Use Low Power Mode on your Apple Watch - Apple Support, updated September 2025. Accessed 16 May 2026.
• Track your AFib History with Apple Watch - Apple Support, updated September 2025. Accessed 16 May 2026.