Wearables and temperature tracking – the whole story

Several of the biggest names in wearables, including Fitbit, Oura and Whoop, have added temperature tracking to their devices.

And rumors suggest that Samsung’s next Galaxy smartwatch and the Apple Watch Series 8 might be next. But what are the benefits?

Wearable thermometers — also known as continuous thermometers — have been around for years in hospitals and medical settings.

“Temperature tracking wearable devices can continuously and seamlessly collect data and record temperature trends,” Professor Hossam Haick, an expert in nanotechnology and non-invasive disease diagnosis, tells us.

“They automatically provide notifications when physician-specified high-temperature thresholds are exceeded.”

But outside of medical care, what’s the point of tracking your temperature 24/7? Why would we want a temperature sensor on our Apple Watch? We found out.

Core body temperature vs skin temperature

Measuring your temperature is confusing because it isn’t consistent throughout your body.

We spoke to Dr Chris Tyler from the University of Roehampton, who specialises in human responses to extreme hot and cold environments.

He says that the holy grail of temperature measurement is hypothalamic temperature—that’s the temperature of your brain. But taking the temperature of your other major organs will provide the same high accuracy. This is your core body temperature.

However, measuring the temperature of your brain isn’t easy in a lab setting, let alone anywhere else. That’s why the most common temperature measurement is your peripheral body temperature.

That is usually measured from the mouth, ear, armpit or rectum. However, all of those places won’t provide the same readings. “The more indirect the method, the less accurate it is,” Dr Tyler said.

We can also measure skin temperature, which is sometimes lumped into peripheral body temperature but tends to be even less accurate than other methods.

“Skin temperature and body temperature can share a relationship.” But he explains that because skin temperature measurement is external, it can be influenced by your environment.

“All you need to do is wear a hat, go for a walk or stand in a breeze for a few minutes, and the relationship is distorted.”

And when it comes to wearables that live on the skin, that presents an accuracy challenge. And we've also seen issues with usefulness. Some budget smartwatches from the Amazfit GTR 3 and Huawei Watch GT 3 have included a skin temperature feature. However, these have been simple temperature readings, with no context, long term averages, or any way for users to understand the data. In short, pretty useless.

However, there have been more innovative approaches.

Finding your baseline

A number of newer Fitbit devices – the Fitbit Sense and Charge 5 (and Charge 4) – measure your skin temperature throughout the night to determine your average temperature.

After three nights, the Fitbit app will show you a personal skin temperature baseline and a personal range of variation. Instead of producing a skin temperature number, which Dr Tyler says bears little resemblance to your core temperature, the wearable will look for deviations from what's normal.

“Your baseline is used to provide insight into when your skin temperature is higher or lower than your baseline and if it is within your personal range, as it is natural for your skin temperature to vary night to night,” Dr Conor Heneghan, of Fitbit says.

You can then look for any temperature changes outside of your personal range.

“This can help you understand changes in your body, like the start of a new menstrual phase, or you could discover potential signs of a fever or illness,” Dr Heneghan explains.

“Skin temperature shouldn’t be used as a substitute for a thermometer that measures core temperature,” he tells me. “It can, however, function as a warning sign that something has changed.”

Both the Oura Ring 3 and Whoop 4.0 take similar skin temperature readings. This data is combined with everything else they know about you to make recommendations and better inform how ready you are to take on the next day.

Interestingly, the Oura ring has been used in a Covid-19 study called TemPredict. The results suggested that the temperature data from the Oura ring – combined with all of the other data it collects – could help to identify the early signs of a Covid-19 infection in participants 2.75 days before they felt ill enough to take a test.

These findings are exciting for the future role wearables might play in tracking disease and illness. But we still shouldn’t treat our wearables like doctors, they might give us a great “warning sign” but not the whole story.

Menstruation and ovulation tracking

There’s huge potential for temperature data to be used to predict menstruation and ovulation, as body temperature increases slightly when ovulation occurs. You can read our piece on femtech and women's health wearables – and watch a talk at the Wareable Presents event below.

In 2019, Oura conducted a small pilot study into the efficacy of menstruation and ovulation tracking using temperature data from the ring. The results were promising. Menstruation was detected with a sensitivity of 71.9 – 86.5%, and ovulation was detected with a sensitivity of 83.3%.

Fast-forward to 2021 and the company has since launched a beta feature called Period Prediction. For people who menstruate, this combines manually entered data about their period’s start and end dates and temperature data to predict with greater accuracy when upcoming periods will occur.

Oura isn’t focusing on ovulation at the moment. But for wearable company Ava, ovulation and fertility tracking is its primary focus.

Many people currently track their fertility by using a thermometer first thing in the morning to find their basal body temperature (BBT). A slight temperature increase is usually a sign of ovulation.

But Ava is a more intelligent and straightforward solution. It takes a skin temperature reading from your wrist as you sleep, and then you see a fertility prediction from the app when you wake up. And was used in Covid-19 detection trials in 2020.

We’ve already learned that skin temperature isn’t the most accurate form of temperature measurement—especially to accurately detect fever and heat-related illnesses. But a 2021 study suggested that it might be more sensitive than BBT specifically for the purpose of ovulation tracking.

Ava combines temperature measurements with data about resting heart rate, heart rate variability, breathing rate, and skin perfusion.

“We were the first ones to look at the effect of multiple physiological signals measured continuously throughout the entire menstrual cycle,” Lea von Bidder, the co-founder and CEO of Ava, told Wareable.

She explained that research previously tracked different signals in isolation but not all of them and not continuously. “Wearable technology allows for much more comprehensive measurements and advanced analysis,” she said.

Ava’s machine learning algorithms then use all that data to detect a five-day fertile window—which Lea von Bidder says is 90% accurate. It’s also much more helpful, as von Bidder tells us other methods identify the date of ovulation, which marks the end of the fertile window.

While many mainstream wearables offer menstrual cycle tracking, few have employed skin temperature analysis to do it. Most implementations use self-input data to predict when the next cycle will be – and not using biometric data.

The future of temperature tracking

When we asked the experts about the future of wearable temperature tracking the answer was the same: improved algorithms.

Several recent studies have explored how algorithms could be used to predict someone’s core body temperature from only a skin temperature reading.

But we’re not there yet. Dr Tyler told us he’s currently working with a company to improve their algorithms and draw in different data points – heat flux and heart rate – to make temperature readings as accurate as possible.

“Currently the predictions are only good enough within pretty small (and stable) ranges,” he tells us.

Ava’s future focus is on rolling out its algorithms to more hardware.

“We are actively exploring the possibility of making Ava’s technology available across other wearables,” von Bidder tells us. “This will help us make it more accessible.”

Issy Towell, a wearables analyst at CSS Insight, told us that she hopes similar temperature tracking wearables will support women at even more stages of their life, including those who are post-partum or managing symptoms of endometriosis or menopause.

“The possibilities for women’s health applications are potentially life-changing for many women around the world if skin temperature readings can become accurate enough to help women understand what’s happening in their bodies,” she tells us.

But she points to a common wearable tech problem that might stand in the way – battery life.

“Part of the battle is making wearable devices that can be worn all the time so that there’s enough data to generate a robust personal temperature baseline,” she said.

Should we get excited about temperature tracking?

With hardware and software improvements, temperature measurement could become more accurate and more useful.

But does anyone need to track their temperature with a wearable right now? It depends.

If you’re looking for a fertility tracking solution, Ava presents a clear use case that’s an improvement over other traditional methods.

For everyone else using their smartwatch, ring or activity tracker for general temperature tracking, the benefits are less clear. Spotting when you’re ill is an obvious application, but how useful is that data, when we’ve established that changes in temperature tend to lag the symptoms.

The data will be fascinating to those interested in discovering more about their health and fitness who enjoy spotting patterns.

For others, the information might be a bit pointless until it’s packaged up in a way that’s more meaningful and actionable. “The problem with providing consumers with vast amounts of skin temperature data is that not all consumers want to become data science hobbyists,” Towell says.

Until then, there’s nothing wrong with using a wearable to track your workouts, heart rate, steps and temperature too.

But we need the algorithms and analysis to match the sensor data – and until then we won’t enjoy the fruits of this new wearable revolution.