What Does Your Sleeping Wrist Temperature Tell You?

Watches and rings that measure your skin temperature overnight report it as a change from your own baseline, such as +0.4 °C, rather than as a temperature. That’s because skin temperature isn’t body temperature: it depends on the room, your bedding, and blood flow to your skin. What the device can see well is how tonight compares with your usual nights, and that change follows your menstrual cycle, illness, alcohol, and more.

This article is general information, not medical advice.

How It’s Measured

The sensor sits against your skin and samples temperature while you sleep, when you’re still and conditions are most alike from night to night. Apple Watch, for example, needs about five nights of sleep tracking to establish your baseline, and then shows each night as a deviation from it; Apple notes that wrist temperature varies with diet and exercise, alcohol, the sleep environment, the menstrual cycle, and illness.

What It Follows

• The menstrual cycle. After ovulation, progesterone raises body temperature for the rest of the cycle. In a pilot study by Maijala and colleagues, 22 women’s nighttime finger skin temperature from a ring was 0.30 °C higher in the luteal phase than the follicular phase, a slightly bigger difference than their oral temperatures showed. An algorithm detected 83% of ovulations within a window from 3 days before to 2 days after the ovulation a urine test confirmed. The study was small, and such estimates are retrospective: they say when ovulation probably happened, not when it will.
• Illness. Smarr and colleagues showed in 2020 that a ring’s skin temperature rose around the onset of COVID-19 symptoms in many of their participants and tracked self-reported fever.
• Alcohol, late meals, and hard evening exercise, which tend to raise it.
• A warmer room or heavier bedding, which can shift it without any change inside you.

How to Read It

• Look at runs of nights, not one night. A single night half a degree up after a warm evening means little; several nights up, with a higher resting heart rate and breathing rate, more often means you’re coming down with something.
• Don’t compare it with a thermometer. A skin deviation of +0.5 °C isn’t a fever of 37.5 °C.
• For cycle tracking, the shift after ovulation shows as a step that lasts until your next period. Basal body temperature, taken by mouth on waking, is the traditional way to see the same step; see What Can Basal Body Temperature Tell You About Your Cycle?

Following It in Kaizora

Sleeping Wrist Temperature comes in with your Apple Health export, on the Vitals page, and it also appears on Menstrual Cycle’s chart of cycles aligned by cycle day, where the luteal step is easy to see. Each night’s value is the wrist temperature Apple Health exported, so compare it with your own other nights rather than with a thermometer. It’s kept apart from Basal Body Temperature, since skin at the wrist reads cooler than the mouth. Kaizora makes no predictions of ovulation or your next period from it. See Importing From Apple Health.

The Short Version

• Sleeping wrist temperature is a change from your own baseline, not a body temperature.
• It rises after ovulation and with illness, alcohol, and a warm bedroom.
• Read runs of nights alongside resting heart rate and breathing rate.

Sources

• Apple, Track your nightly wrist temperature changes with Apple Watch.
• Maijala A et al., “Nocturnal finger skin temperature in menstrual cycle tracking: ambulatory pilot study using a wearable Oura ring,” BMC Women’s Health 19 (2019): 150. doi:10.1186/s12905-019-0844-9
• Smarr BL et al., “Feasibility of continuous fever monitoring using wearable devices,” Scientific Reports 10 (2020): 21640. doi:10.1038/s41598-020-78355-6