The new Apple Watch's biggest feature is not, for the most part, visible to anyone who is wearing it. Every so often, in the dark, your wrist might glow red, reflecting a light shining out of the Watch that is measuring your blood.
The Watch uses that glow to measure the amount of oxygen in your blood. And, Apple hopes, that measurement could provide entirely new ways of understanding your own wellbeing.
That red glow comes from LEDs on the back of the watch, which shine out, through your skin and into the blood. A sensor made up of photodiodes, in the same panel, can measure the amount of light that's reflected back – the brighter the blood, the more oxygen it is carrying – feeding those measurements into algorithms that provide an estimate of blood oxygen.
When you head to Apple's page for its new Watch, it's not the speed of the processors or the brightness of the screen that it tries to sell you on. Instead, there's that same red glow, and the promise that "the future of health is on your wrist".
Apple hasn't always talked about its Watch this way. When it first arrived, in 2015, it wasn't necessarily particularly health-focused at all.
Early marketing focused on three main features: its function as a way of telling the time, its various communication features, and its role as a "comprehensive health and fitness device". But that was weighted much more towards fitness than it was health, and the heart rate monitor that was included on its back was put there primarily as a way of ensuring that it could get an effective measurement of calories, rather than offering any view of the heart itself.
Over time, that changed. With the introduction of the Series 4 in 2018, Apple added the ability to collect an electrocardiogram or ECG, a graph showing the precise workings of a person's heart. It received clearance from the US Food and Drug Administration – the first time a smartwatch had done so – and Apple stressed that while many people might never require the feature, it could be vital for those who do.
Over time, Apple highlighted some of those: its keynotes began to feature messages from people who had been alerted by the heart rate monitor to unusual readings, whose Watches had detected a fall and allowed them to call for help, whose ECGs had shown irregularities. Those features – largely living in the background, and focused on people's wellbeing – became a key part of Apple's presentations. Measurements of the speed of a processor or battery life being displaced by measurements of heart rate and cardiovascular health.
Tim Cook said in 2019 that – when people look back from an imagined future at the work his company did – he believed Apple's greatest contribution to mankind would be "about health". It was a bold claim for a company that prides itself on fundamentally having changed the way people communicate with the iPhone, and other advances, but if nothing else it was an indication of where Mr Cook's priorities are.
The new Series 6 is surely the most clear example of that yet. While the Watch does have other advances and new features that are of the kind you would normally expect from a technology company – a faster processor, a brighter screen – it's the blood oxygen sensor that dominated the introduction, and which is the new feature that Apple has spent the most time talking about.
It's a change that has happened gradually over the now relatively long life of the Apple Watch. But the company is aware of its particularly topical nature at the moment, as the entire world grapples with a health crisis unprecedented in living memory.
"We’re living in a time with a constant reminder that there's nothing more important than your health," says Jeff Williams, Apple's chief operating officer. "At Apple, our health journey happened organically. It happened as we pulled on threads at different points.
"We have an opportunity and a huge responsibility as a matter of fact to help people and empower them with more information about their health.”
Other companies have added blood oxygen sensors before: Garmin, for instance, has incorporated it in many of its high-end watches and uses it to inform indications of whether its wearer has adjusted to changes in altitude and how much that might affect their exercise.
But the metric has never enjoyed quite such a mainstream presence, being promoted by the biggest company in the world on the biggest watch in the world.
For now, the uses of the blood oxygen sensor are somewhat limited. There's not all that much a person can do about the amount of oxygen in their blood, and so it is better described as a measure of a person's wellbeing than a target to improve like some of the other metrics that come out of the watch.
"Blood oxygen saturation reflects both the environment you’re in - whether you’re at high altitude for example - and how well your body is able to take oxygen in and deliver it through the blood," says Williams. "It is a powerful signal about you and your wellbeing."
But pulse oximetry is an investment for the future. As a measure of how well the blood is doing its job, it could be a key metric for predicting, sensing and living with a variety of medical issues.
Those people who buy the new Apple Watch are participating in work that is heading towards that. During the reveal of the watch, Sumbul Desai, Apple's vice president of health, announced that the company was not only integrating the blood oxygen sensor but launching three new studies to understand how it might be used.
"It is meant for fitness and wellness purposes," says Desai. "We're doing research to learn more. And backing what we can do with science is really important which is why we take the approach of research and want to go on this journey together with both our users and the medical community.”
That research includes an asthma study working with University of California, Irvine, and Anthem and another looking at the relationship between metrics including blood oxygen and heart failure, working with the University Health Network and the University of Toronto. But perhaps most exciting for the future – and certainly most topical – is a study working with the Seattle Flu Study and the University of Washington School of Medicine to see whether heart rate and blood oxygen can be used to spot respiratory conditions including flu and covid-19.
There have already been indications that pulse oximetry could be used to spot such diseases, and a range of technology companies are working to understand whether their blood oxygen sensors might have any role in the fight against covid-19. But again Apple's scope – of its products, the number of its users and its ambitions – could rapidly accelerate that research, meaning that the sensor that is used for wellness today could one day become even more, precisely as a result of you buying and wearing it.
Blood oxygen isn't the only new feature that aims to bring what has historically been a specialised and medicalised measurement onto the watch. With new updates, Apple has added the ability to measure VO2 max in a lower range, which it says will be useful for those people who are using their watch to bring their fitness up from a lower level.
VO2 max is a fairly specialised metric: in its strictest sense, it measures the maximum amount of oxygen that a person is able to take in. But it has also been used a test of a person's cardiovascular fitness, and it's that more general indication of health that the Apple Watch aims to recreate.
Historically, the VO2 max test was a fairly gruelling endeavour: a participant straps on a mask that monitors their breathing while they undertake intense exercise such a running or moving on an exercise bike. The Apple Watch has been able to recreate that by watching its wearer's heart rate as they exercise, inferring their fitness by measuring how much a given activity requires of their cardiovascular system.
"The thing that's really powerful about VO2 Max is that you traditionally have to go into a clinic," says Desai. "It's a really uncomfortable test to do. Now to be able to measure that with watch will be really powerful.”
Now the Watch will be able to expand that metric, giving it to even more people. And the utility of the readings is that – perhaps unlike blood oxygen – people can very much do something about this, since improving their fitness will improve the measurement, and what they do will be reflected in that reading over time.
"Apple did a huge body of work to measure lower VO2 max range," says Williams. "Now it’s easily available, I see a future where this is a much more common and meaningful metric of health. And the great news about this one is that you can actually take action and improve it. It’s not just a signal that you’re in a low range, you can change the trajectory of.”