SAN FRANCISCO (KGO) -- You see them everywhere: rubbery wristbands and high tech watches that track heart rate and exercise, worn by an estimated 1 in 4 Americans.
"I've got one at home," said Michael Garrison, who we found eating lunch at a Mexican restaurant on 2nd Street. "It's just, it's dead, so I don't have it on today."
Sarah Cragin, waiting in line for a burrito, told us her friend was obsessed with the new tracker she'd gotten, and wore it religiously. Until she didn't.
"I think it was kind of like a phase," Cragin said. "Just a couple weeks or so, maybe a couple months."
Whether it's an Apple Watch or a FitBit, CNET's Vanessa Hand Orellana says frequent charging is just one point of dissatisfaction with the devices.
"Another could be fashion," she said. "They're not the most attractive."
But in an interview overlooking the bustling burrito shop, Spire CEO Jonathan Palley made clear that he fancies himself a bit of a sorcerer.
"We've made the device disappear," he said with a twinkle in his eye.
Holding a pair of blue and green plaid boxer shorts, he pointed inside the waistband.
"It's right there," he said.
Stuck to the elastic waistband with a thin layer of adhesive, the tiny, cloth-covered device is called Spire HealthTag. It's washable, and it measures movement, heart rate and breathing patterns, so it knows when you're stressed.
"Our breathing gets faster, more erratic," when we're stressed, Palley said, demonstrating with an audible gasp. "We also measure a good kind of stress, which we call focus."
The app can tell you when to stop and take a deep breath, or when a little exercise right now could help you get better sleep later. HealthTag is meant to be worn day and night, and sticks fine to flannel pajamas, but Palley say it's most likely to be worn on bras and boxers.
"We've actually done a lot of research on people's underwear habits," he said with a nervous laugh.
Palley said they found most women own 6 to 8 bras that they wear regularly, and most men have 8 to 10 "good" pairs of boxers -- not counting the "backup" pairs stuffed in the back of the underwear drawer. So Spire expects many customers will opt for an 8-pack of tags, which are now on sale for $200. A single tag costs $50 -- an option Hand Orellana said is a smart move.
"For something as new as this, I think people are gonna start off with one -- they want to try it out," she said, and added it won't replace a smartwatch for those who already use one. "Having a screen on your wrist with that information is really helpful at least for me personally."
But unlike a smartwatch, HealthTag doesn't need to charge. The internal battery lasts a year and a half and is recycled when you trade in the tag for a discounted replacement at the end of its lifecycle.
"It would be way better if I didn't have to charge it," Garrison lamented of the Apple Watch that sat at home on his nightstand. "It would be way better if I didn't have to charge anything."
Wearable health tracker "disappears" into your clothes