Skin Temperature as a Digital Biomarker

Skin temperature is the temperature at the surface of the body where the sensor touches it, usually the wrist or finger. It is not core body temperature, and the distinction is the single most important thing on this page. Core temperature is tightly regulated; skin temperature is a product of core temperature, peripheral blood flow and the surrounding environment, and the environment often dominates.

Peripheral skin temperature also carries genuine circadian information. It follows a daily rhythm driven by changes in peripheral vasodilation, rising in the evening as the body prepares for sleep and falling towards morning, and wrist temperature has been proposed as an index of the circadian system on that basis.

Used as a relative measure against a person's own established baseline, it detects meaningful physiological change. Used as an absolute reading, it is not a thermometer.

Wearables use a thermistor or infrared sensor in contact with the skin, sampling continuously or at intervals. Most consumer implementations report a deviation from the wearer's own recent baseline rather than an absolute value, which is a deliberate and correct design choice given how much ambient conditions affect the raw reading.

Nocturnal measurement is the most usable window, for the same reason it is for heart rate: the person is still, indoors, under bedding, and the environment is relatively stable night to night. Daytime readings track clothing, weather, activity and indoor heating more than they track physiology.

Establishing a baseline takes time. Most devices need one to two weeks of wear before deviation becomes interpretable, which makes short studies a poor fit for this measure.

Fever and infection detection is the most developed application. Continuous wrist temperature monitoring has been assessed for feasibility as a fever detection method, and large wearable studies during the COVID-19 pandemic used skin temperature as one input to multimodal illness detection, with syndromic surveillance work following.

The second application is circadian and sleep research, where the evening rise in distal skin temperature marks the physiological transition towards sleep and can be used to characterise circadian timing without a laboratory protocol.

The third is menstrual cycle research, where a small sustained nocturnal rise follows ovulation. Across all three, skin temperature is used as a supporting physiological signal rather than a primary endpoint, and it has no questionnaire counterpart at all.

Ambient temperature is a first order confounder, not a nuisance. Room heating, bedding, seasonal change and travel all move wrist temperature independently of the body, and studies that do not record environmental context will misread that variation as physiology.

Absolute values are not comparable between devices or between people, and none of these sensors is a substitute for a clinical thermometer. Sensor placement, fit and skin contact all change the reading.

The measure also requires a stable baseline period, which excludes short studies and makes it vulnerable to anything that shifts the baseline mid study, including a change in living arrangements or season. There is no established threshold for what size of deviation is clinically meaningful.

• Sarabia JA, et al. Circadian rhythm of wrist temperature in normal-living subjects: a candidate of new index of the circadian system. Physiol Behav. 2008. pubmed.ncbi.nlm.nih.gov
• Smarr BL, et al. Feasibility of continuous fever monitoring using wearable devices. Sci Rep. 2020. pubmed.ncbi.nlm.nih.gov
• Mason AE, et al. Detection of COVID-19 using multimodal data from a wearable device: results from the first TemPredict study. Sci Rep. 2022. pubmed.ncbi.nlm.nih.gov
• Kasl P, et al. Utilizing wearable device data for syndromic surveillance: a fever detection approach. Sensors. 2024. pubmed.ncbi.nlm.nih.gov