Nightly skin temperature
The Nightly Skin Temperature measurement automatically tracks your skin temperature when you sleep. It then compares the result with your 28-day average, and shows the variation to that average. Tracking the variations in your skin temperature can help you detect changes in your body's state.
In contrast to core body temperature, which is usually quite stable around +37 degrees Celsius (98.6 °F), the temperature of your skin can vary much more. This is because the body regulates its core temperature through changing the blood flow of the skin. Also, environmental factors affect the skin temperature. A variation of +-1 degrees Celsius / +-1.8 degrees Fahrenheit is considered usual in skin temperature. Lower temperatures can occur quite often due to environmental factors, such as colder room temperature or sleeping with your hand outside the covers. Higher temperatures might be a sign of oncoming illness. Higher temperatures can also indicate being in the latter part of the menstrual cycle (ovulation and luteal phase).
The skin temperature feature can't be used as a thermometer, as it doesn't show any absolute temperature values, only the variation to your average skin temperature. If you're feeling under the weather, double-check your temperature with a thermometer.
The temperature measurement feature and data is not intended for medical purposes, diagnosis or treatment.
- Continuous heart rate tracking needs to be enabled for Nightly skin temperature measurement to function. To enable Continuous HR tracking go to Settings > General Settings > Continuous HR tracking and choose On or Night-time only.
- Tighten the wristband firmly around your wrist and wear the watch when you sleep. For detailed wearing instructions, see Wrist-based heart rate measurement.
- You need to wear your watch for three nights before you start to see your Nightly skin temperature results on your watch. After three nights, we know your average skin temperature and show it as baseline (zero). When calculating the baseline, the past 28 nights are taken into account.
Sync your watch with the Polar Flow app to see your skin temperature data in the app. Note that you must sync your watch using the Flow app. Skin temperature data cannot be synced using the Polar FlowSync software on your computer.
In the Polar Flow app, choose Skin temperature from the menu (1). From the Skin temperature view (2), you can check the result of last night's measurement and also see how your skin temperature varies over the course of a week or month. You can also check the result of last night's measurement from the card in the Diary (3). Tapping the card opens the Skin temperature view.
When your watch detects that you have woken up, it compares the skin temperature measured during sleep to your average skin temperature, and shows the variation to that average. Navigate to the Nightly skin temperature view by swiping left or right from the watch face.
You can also stop the sleep tracking manually if you watch has not yet summarized your sleep. Zzz... is shown on the Nightly skin temperature view when your watch has detected a minimum of four hours of sleep. Already awake? appears when you tap the display. Confirm by tapping 
and the watch summarizes your sleep instantly. If sleep isn't recognized, skin temperature won't be available.
The information displayed on the Nightly skin temperature view includes:
- Variation to your average skin temperature.
- Your skin temperature compared to usual level. Scale: Below usual – Usual – Above usual – Much above usual.
Variation within +-1 degrees Celsius or +-1.8 degrees Fahrenheit is considered Usual. If the variation to your average skin temperature is more than +2 degrees Celsius or +3.6 degrees Fahrenheit, it is considered Much above usual. - A graph showing your skin temperature measurements from the last 7 nights.