You may have heard of the Fitbit or the UP band: $50-ish to $100-ish wristbands that measure your steps throughout the day, like a high-tech pedometer, and display your progress as a graph on your smartphone.

But this product category has exploded well beyond those common names. There's the Nike+ FuelBand, Garmin Vivofit, the Basis Peak, the Magellan Echo, the Misfit Shine, and on and on. Health tracking is also built into the Apple Watch and the Samsung Gear watches. Wearable fitness monitoring has become a $1.15-billion industry.

All these gadgets count steps. Most also measure sleep, revealing fascinating details about the one third of your life that you spend unconscious. The fancier models can also tabulate other metrics, including heart rate, blood oxygen level, skin temperature, perspiration, body weight and body mass.

That's the great appeal. These gadgets allow us, mere untrained mortals, to gauge what only doctors used to measure. We gain knowledge about the workings of our own bodies—by monitoring measurements continuously, not once a year at a physical.

Meet the quantified-self movement. It's a Web site, it's a conference, it's communities of people, some of whom are raising self-monitoring to the level of obsession.

Millions of people making a greater effort to get healthy and fit—who could argue with that?

There are a couple of obvious problems with the mad rush to quantify ourselves, though—and to sell us gadgets for it.

First, we're almost certainly ascribing more precision to these devices than they deserve. If you wear three brands of fitness band, you'll rack up three different step counts by the end of each day. And don't get sleep scientists started on the accuracy of those sleep graphs; according to researchers, it's brain waves, not wrist movement, that indicate what stage of sleep you're in.

But you know what? It doesn't matter. These devices are succeeding not because of their scientific qualities but because of their motivational ones. We all know we should move more and sleep better—but with slow decline, most of us don't bother.

What the fitness bands do is to keep these issues front-of-mind. There it is, every time you turn on your phone: the latest stats on your progress. Most also show the results of friends who wear the same brand; it's fitness through humiliation.

In other words, the accuracy really makes little difference; the point is to keep us aware, to gamify our efforts. In that way, these bands really work. You wind up parking farther away, getting off the bus a stop earlier, going for a walk down the block to bring your 9,374 daily step count up to your 10,000-step goal.

The other concern is less easily dismissed: the data. Terabytes of personal health data, amassed daily in stunning quantities. It's the world's biggest health study—and nobody's running it.

Researchers would love to get their hands on that information. So would advertisers. Insurance companies would have a field day; they could offer active members lower rates than sedentary sloths. (Our rates are already higher if we're smokers or drivers with bad records.)

Who owns the data? Will the makers of the fitness bands sell personal information? Will it be anonymous and aggregated or associated with us by name? What if we want to contribute our data—to a doctor? To a research study?

It's the Wild West at the moment. We're collecting mountains of personal health data and just shoving them into underground caverns. The real promise of the quantified-self movement may not be fulfilled until we determine how to find the gold in those data—and who gets to do the looking.

Health-monitoring socks and more: