Skeptical but curious – that's how I felt strapping on my first activity tracker, a Misfit Shine. All of these devices, from the Fitbit Flex to the Jawbone UP24, promise to get a man fitter, better rested, and lighter around the middle. I'm not the leanest guy, and I don't sleep like I used to, so I wanted to believe. And I'd heard hype like this: One in 10 American adults owns a body monitor, and retail sales – $290 million in 2013 – are projected to double this year. But I also exercise plenty, and I couldn't see how a gadget counting my steps and hours in bed – what most of them do – would change my life. But then my little tracker started spitting out revealing data, and friends started sharing their own experiences. Soon, I was upgrading and narrowing in on the exact-right device, and becoming convinced that, to make those health promises a reality, it only takes a little knowledge about how these trackers work, what they're good for, and your own motivation.

Pick Your Goal for a Plan and Ideal Tracker:

1. You Want to Move More, and Lose a Few

2. You're a Data Junkie Who Wants to Dial Up His Fitness

3. You're a Wannabe Pro

1. You Want to Move More, and Lose a Few
If you don't have time for the gym but you want a nudge to move around more – or if , like me, you're curious to find out how active you are during the 23 hours a day you're not exercising – basic activity trackers are the way to go. From the Misfit Shine to the Fitbit, most depend on simple accelerometers that measure motion in three dimensions. A computer algorithm uses data from that accelerometer to make rough guesses about how many steps you've taken, how many calories you've burned, and how much time you've spent lying still – and, therefore, likely asleep.

Mobile apps and websites encourage you to set and pursue goals based on these numbers, like walking 10,000 steps a day, the minimum recommended by the American Heart Association. "That's super helpful for people looking for motivation," says Dr. Jordan Metzl, sports medicine physician at New York City's Hospital for Special Surgery. "It gives people some sense of what being active looks and feels like." Metzl says it can also help boost your so-called NEAT profile – your non-exercise active thermogenesis, meaning the calories you burn in everyday life.

The downside: These devices can't tell the difference between sitting on the couch and squatting 500 pounds, but, says Misfit founder and CEO Sonny Vu, that's not the point. "Basic trackers are for people who don't care about the details. They just want to know, 'What's a win, for me, today?'?"

In my own case, after discovering that I'm shockingly sedentary between workout sessions, it meant simply parking a little farther away from the kids' school, or heading to the grocery store on foot now and then. For my friend Alex Hochman, a University of San Fran­cisco career counselor who bought a Fit­bit last year, it meant chasing that 10,000-steps-a-day number with such success that it helped him lose 40 pounds in four months. "I feel naked now if the Fitbit isn't on me," says Alex. "When I get dressed, it's on my belt the rest of the day. My kids tease me, but when you're trying to lose serious weight, it helps you reach those incremental goals."

Your Tracker: Misfit Shine

The waterproof Shine uses a long-lasting battery that never needs to be recharged, and it's adjustable so you can wear it on your wrist while swimming, your ankle while spinning, or on your belt for walking or running. It also tracks how many hours you sleep.

2. You're a Data Junkie Who Wants to Dial Up His Fitness
It turns out there's a subcategory of activity trackers, including the BodyMedia Armband and the Basis health and heart-rate monitor, that pack in a bunch of extra sensors to measure steps, calories, and sleep with far greater accuracy than simpler devices. I picked up a Basis and quickly got interested in its hyper-precise sleep readout, showing exactly how little mind-restoring REM sleep I was getting every night. After a month testing the watch, I also discovered that my temperature was low, as well as my calorie-burn tally, during my longer runs, which indicated I wasn't pushing myself as hard as I thought I was. You can also use these souped-up monitors to measure the effects of overtraining: for example, checking your resting heart rate when you wake up. "There is almost no better way to measure fatigue," says Steven Devor, a professor of exercise physiology at Ohio State University. "If you're typically at 60 beats per minute, and you see it's 66, you know you need to take a rest day."

I was impressed to hear about the results Sami Inkinen, a 38-year-old triathlete and co-founder of the online real estate company Trulia, saw from a body monitor called Beddit, a European device slated to hit U.S. markets soon. Persistent tiredness and lackluster race results suggested that he wasn't bouncing back quickly enough from workouts. The Beddit, a thin strip that slips underneath a sheet, measured each night's mind-restoring REM sleep, body-restoring deep sleep, how often he tossed and turned, and his resting heart rate.

It also charted Iniken's heart-rate variability, a widely-accepted marker for physiological stress, noting when a steady pulse indicated he was relaxed and when rapid fluctuations showed he was strained.

The Beddit's sensor (which uses your smartphone's microphone) detected sound levels in Inkinen's bedroom, too. "It helped me see that I was waking up in the middle of the night, every night, around the time sound spikes from street noise," Inkinen says. He adapted first by closing windows, then by tinkering with other factors known to affect sleep – cutting back on late-afternoon coffee and after-dinner booze, and establishing a highly regular sleep schedule.

The effect data tracking has had on Inkinen's performance has been dramatic: He became overall amateur champion at Ironman 70.3 Hawaii. He's told me since that he's made micro-monitoring his sleep a permanent part of his life. "It lets me see exactly how stressed my body is every morning, so now if it seems like things are falling apart, I might make that day's workout easier, or cancel a dinner or a meeting and make sure I get to bed earlier."

Your Tracker: Basis health and heart-rate monitor

The Basis is a multisensor that gauges, minute by minute, everything from skin temperature spikes to how hydrated you are. Best, it provides sleep metrics – such as hours spent in REM and how often you wake up – of near-clinical accuracy, according to experts.

3. You're a Wannabe Pro
This is me. And if you, too, love beating last-year's time in your local trail run, ramping up for a triathlon, training for an open-water swim, you'll want to check out sports-focused body monitors, like those from Garmin and Polar, which now produce a mind-boggling range of features. Different models aimed at various sports mix and match from a laundry list of sensors that include GPS, heart-rate monitors, accelerometers, gyroscopes that measure force, and magnetometers that measure rotation. Onboard chips crunch the data to produce athletic metrics useful to everybody from mountain bikers to snowboarders to marathoners.

Endurance athletes have long used the hallmark metric of this category – heart rate – to establish so-called training zones, heart-rate brackets that produce different fitness effects, like expanding your aerobic capacity or building your tolerance for lactic acid.

The latest generation of these devices can do so much more than track heart rate: for instance, telling me how many strokes I've taken in one length of the pool (a measure of efficiency) and even my running metrics, like ground-contact time (the milliseconds my feet spend on the ground between strides).

"If you're implementing that data correctly, it can tell you a lot about your stride – if it's too long, too short, what your mechanics are like," says Devor. That's what helped me: After years of painful running injuries, I've found a Garmin watch that allows me to monitor – in real time – running metrics related to risk of injury, including stride cadence and average vertical oscillation, meaning how much I bounce up and down with each step. By working to improve each metric, I'm now running faster than ever, without pain. My pick is the the Garmin fenix 2. Suffice it to say you won't find it in my sock drawer any time soon.

Your Tracker: Garmin fenix 2

There's little the fenix 2 can't do to better your sport. Our two favorite features: It crunches numbers such as your height, weight, and pulse to tell you how well you've recovered since your last workout, and it can estimate your V02 max to predict your best possible time for any running distance.