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ZDNET's key takeaways 

• Health trackers are getting smaller. 
• They're also harder to recognize. 
• This design change reflects health technology's vision.  

If 10 years ago, you wanted to know whether the people around you were tracking their health, there would be some dead giveaways. You could check their wrists for an Apple Watch, Fitbit, or Nike Fuelband. Today, it might be harder to tell. Sure, smartwatches and smartbands are alive and well, but a multitude of other designs have entered the market. 

Continuous glucose monitors (CGMs) are hidden in shirt sleeves. Smart rings, earrings, bracelets, and even necklaces blend in with regular accessories, and fitness bands disappear against neutral fabrics to match an outfit. The makers of these discreet trackers want them as invisible as possible. 

Also: What you give up when you put on a smartwatch or ring

Through advances in hardware and software, companies are building the next generation of wearables that are even lighter, smaller, more capable, and less visible than the previous generation. 

"Over time, we've noticed that these products have gotten smaller," Forrester principal analyst Arielle Trzcinski said about health wearables in an interview with ZDNET. 

Tech companies have always been in the business of optimizing for size. Apple's first MacBook weighed 5 pounds. The latest model weighs half that. But while phone companies are shipping bigger smartphones with massive screens and trifoldable designs, the accessories that connect to these phones have miniaturized. 

So, how did these devices go from bulky and branded to indistinct and invisible? And why? 

Honey, we shrunk the health tracker

When Tim Cook unveiled the Apple Watch in 2014, he jump-started the burgeoning mobile device category and a new way to interact with your phone (this time, by having its companion around your wrist). It had a distinct, rounded square design that was quintessentially Apple. 

The smartwatch was easy to recognize, and it became a conversation topic in its infancy. As more competitors entered the market, they distinguished themselves by their bold designs.

I don't remember the last time I was gobsmacked by a smartwatch or compelled enough to start a conversation about it. 

Also: How I used Airtable to swap my daily fast-food habit with 5-minute meal planning

"Usually, when products come to the market, including the Apple Watch, they are designed so that they can be recognized," Khosravi said. Over 550 million people worldwide own a smartwatch, according to DemandSage data. Tech companies no longer have to sell consumers on the value of tracking their sleep, steps, or stress, nor the positive health outcomes of doing so with a wearable. We're already sold. 

Beyond smartwatches, even the smaller trackers are getting tinier. While Oura wasn't the first to introduce smart rings as health trackers, it was the one to take this design mainstream and sell us on discreet devices we could use for sleep tracking. Its bet on a near-invisible build has paid off; in September, Oura announced it had sold 5.5 million Oura Rings. It also recently and confidentially filed for an IPO. 

In late May, Oura unveiled the Oura Ring 5, its smallest smart ring yet, 40% thinner than the Oura Ring 4. Reducing the size involved miniaturizing the LEDs that track health metrics and changing the battery. While it slimmed down the Ring 5, Oura also increased the battery life -- from five to eight days to six to nine days. 

The combination of more powerful LEDs, a better battery, and Oura's refined algorithm allowed the 5th-generation ring to deliver more power with a slimmer design, Oura VP of product Maz Brumand explained to ZDNET. 

"My bet is that, after this ring comes out, it's going to be very hard to recognize that this is actually an Oura Ring. People might say, 'Don't you want people to know that someone is wearing an Oura Ring?' That's nice, but the goal or the mission is to fit into people's lives the way they want," Brumand said. 

Also: I should've listened to my Oura Ring when it warned me about my health

Companies are building smart jewelry with recognition as an afterthought. Take the Lumia smart earrings, for example. Lumia's smart earrings track blood flow and attach to the back of an earring stub. The device's earring back can be swapped with any earring stub, making the product extremely inconspicuous. 

But it's not just consumer health tech that's shrinking. Diabetes management and CGM maker Dexcom announced in May that it is reducing the size of its latest CGM by 50%. 

"They are trying to make these wearables in a way that is more invisible and easier to integrate into our lifestyle," Safoora Khosravi, senior research associate at Lux Research Inc., told ZDNET. 

Once they're worn consistently, they can reveal more useful, behavior-changing health information. A fuller picture of behavior, activity trends, sleep patterns, and diet emerges over time as a person wears a health tracker and logs these data points. With more recorded data, a device can more accurately spot deviations or diagnose conditions, as is the case with Apple's sleep apnea, hypertension, and atrial fibrillation detection. 

But wearers are also learning more about the physiological effects of their habits, like that nighttime glass of wine on their sleep and heart health, by wearing a tracker to bed each night.

The build of these devices reflects the mission these tech companies are slowly but surely inching toward. Create something that can be worn all the time, diagnose or detect conditions with FDA-cleared features, connect with doctors when necessary, and build a big-picture view of health through a small, always-worn device.

Small device, big job 

Another key reason why these devices are smaller and more discreet is actually quite simple. 

They don't need to be big to do their job.  

The majority of these devices work in the background. Health trackers record data on the device, send it to the app, and the software sifts through it to create a comprehensive health summary that a user can review and act on. 

Also: Wearables produce huge amounts of health data - and doctors are struggling to keep up

A health tracker is most useful when it's passively monitoring in the background -- with a passive, indistinct build to boot. That explains why many modern health trackers don't call as much attention to themselves -- or even look like them in the first place. 

Data powers all these revelatory diagnostics, and most of the time, it does so retroactively. Unless a user is logging a workout or taking an instant heart rate reading, which requires immediate processing and information display, that data transfer doesn't need to happen automatically, Khosravi explained. Storage takes up a small part of the device. "They don't have to have the hard burst for analyzing the data. They just have to send the data to the phone," Khosravi said. 

While these health technology products are sold on the premise that they could alert you of a heart attack or dial 911 for you in the event of an emergency, Trzcinski called that an edge case, one of the few cases where a user must be alerted in real time about their health. 

Also: The biggest risks lurking inside your at-home DNA and health tests

This stands in stark contrast to AI wearables like smart glasses or pins. They take up more space on the face or body, Trzcinski explained, because they solve an in-the-moment problem. Smart glasses can translate languages, provide real-time AI assistance, take photos or videos, and play audio. That requires more computing power than recording heart rate or body temperature and sending the data to a phone. 

The magic happens on the app tied to the device, not the actual device, Trzcinski said. "The value you're getting is from the app," Trzcinski said. The software on these apps that digests this data and presents it in a helpful, useful, or even diagnostic way is the key reason people are using them.

Tech companies have uncovered the secret to successful health trackers: These devices come in small packages to do the big job of synthesizing lifestyle information or spotting health anomalies. They must be discreet and easy to wear to stay on the body for as long as possible. 

"Now wearables are just trying to embed into the user's daily life," Khosravi said.