# Overnight temperature deviation

How far skin temperature sat from your own baseline, never an absolute temperature. Rises before a fever, with alcohol and late meals, and in the second half of the menstrual cycle.

Type: biomarker · Unit: ±°C · Domain: Cardio-Respiratory Health · Accuracy: Trend-grade · Importance: Core

Also called: skin temperature, body temperature deviation, temperature trend, nighttime temperature, Oura temperature

## In one sentence

How far your skin temperature sat above or below your own usual last night, in degrees, which is the only form in which a ring's temperature is worth keeping.

## Why it is a deviation and not a temperature

A ring or band reads the skin of a finger or wrist, which is cooler than the body's core and moves with the room, the bedding and the blood flow to the hand. As an absolute figure it means little; as a change against the same person's own nights on the same device it is one of the most sensitive readings a wearable takes, because the core temperature's rises and falls reach the skin. That is why VT stores it only as a deviation from your own baseline, and why it is a main reading in Symptom Radar.

## What moves it

Upward: a fever, which reaches the skin as the core rises; the second half of the menstrual cycle, by about 0.30 degrees in nocturnal finger temperature across 22 women tracked for about four months (Maijala 2019), a biphasic pattern confirmed in 26 women in a laboratory study (Alzueta 2022); an evening of alcohol, which opens the skin's blood vessels; a hot room, a heavy duvet, a late hard session, a late large meal. Downward: a cold room, an open window, a loose ring. In women the cycle is the largest regular mover, which is why VT keeps the cycle phase beside the reading.

## How to measure it well

The same device every night and, as far as life allows, the same bedroom; a change of bedding or season shifts the baseline for a while, and a holiday somewhere hot will show up for a few nights until the 30-night baseline catches up.

## How Vitality Tracker uses it

A main reading in Symptom Radar (weight 1.0, higher is the direction it watches), compared with your own 30-night baseline, with your period and a large late meal taken into account before anything is said. Symptom Radar does not yet start a new baseline for this reading by itself when you change device, so expect a few unsettled nights after a switch while the baseline catches up. Once Symptom Radar is running, a deviation of more than 1.5 degrees in either direction adds a safety note to the morning's result, whatever the baseline. The basal body temperature some members take by thermometer for cycle tracking is a separate field and is not mixed with the ring's deviation.

## How far to trust the number

Trend-grade by construction: the device's own baseline is the reference, and the level has no meaning. The within-person signal is good enough that in Maijala's study the ring's nocturnal temperature alone placed the start of menstruation within a few days in 72 to 87 percent of cycles.

## Method

The device reports the night's skin temperature as a difference from its own rolling baseline for that wearer; VT stores the deviation and compares it with the distribution of your recent clean nights.

## Sources

1. Maijala A, Kinnunen H, Koskimäki H, Jämsä T, Kangas M. Nocturnal finger skin temperature in menstrual cycle tracking: ambulatory pilot study using a wearable Oura ring. BMC Womens Health 2019;19:150. [doi:10.1186/s12905-019-0844-9](https://doi.org/10.1186/s12905-019-0844-9)
2. Alzueta E, et al. Tracking sleep, temperature, heart rate, and daily symptoms across the menstrual cycle with the Oura ring in healthy women. Int J Womens Health 2022;14:491–503. [doi:10.2147/IJWH.S341917](https://doi.org/10.2147/IJWH.S341917)
3. Karjalainen J, Viitasalo M. Fever and cardiac rhythm. Arch Intern Med 1986;146:1169–71 (PMID 2424378).

## Related

- [Overnight resting heart rate](https://vitalitytracker.app/knowledge/overnight-resting-heart-rate)
- [Overnight respiratory rate](https://vitalitytracker.app/knowledge/overnight-respiratory-rate)
- [Symptom Radar and Symptom Trend](https://vitalitytracker.app/knowledge/symptom-radar-and-symptom-trend)

Reviewed October 2026 · v1

Source: https://vitalitytracker.app/knowledge/overnight-temperature-deviation