You’ve heard of the Quantified Self movement? It’s the rise of watches, clips and bracelets that monitor your physical activity, sleep and other biological functions. The idea is that continual numerical awareness of your lifestyle works to motivate you: to park farther away, to get off the subway one stop sooner, to take more stairs. You study the graphs, you crunch the numbers, you live a longer, healthier life.

The most popular such gizmo - or at least the most heavily marketed - has been Jawbone’s stylish, rubberised, shower-proof Up band ($130). For about a week on a battery charge, it quietly measures your movement, whether you are awake or asleep, and displays the results on your iPhone or Android phone.

The trouble with the Up band, however, is the way it communicates with your phone. You remove the bracelet. Pull off a metal cap about the size of a blood cell. Plug the newly exposed connector into - get this - your phone’s headphone jack. Open the app. After the data transfer, disconnect the band, find the cap, snap it back on and put the bracelet on again. No wireless? What is this, 1957? The inconvenience is bad enough, but - well, let’s put it this way: There’s a reason Up sells a three-pack of replacement caps ($10).

Now Fitbit, whose original 2008 tracker clipped to your clothing, has entered the bracelet game with a screamingly obvious improvement: Bluetooth. Its new Flex band ($100) communicates with your phone wirelessly and automatically. You don’t have to remove, dismantle or even touch the band on your wrist. The Flex’s factory setting is to transmit the data only when you open the Fitbit app on the phone, so you sacrifice little phone battery life. (The Nike FuelBand, $150, also uses Bluetooth, though it’s a simpler device; it doesn’t try to track sleep or diet.)

The Flex band makes little attempt to score style points. It’s just a thin, solid gray or black rubber strap, a half-inch wide, thicker as it crosses the top of your wrist. But it’s incredibly light and comfortable. In fact, without looking, it’s hard to remember if you’re even wearing it.

Compared to the Up - which is shaped like an overgrown C, with overlapping ends - the Flex’s design has three advantages. First, it’s a complete circle, so it doesn’t catch on clothing, fly off with your sweater or gouge your hugging partners.

Second, it has a sort of screen. It usually looks like a dark stripe across the band, just a graphic accent. But if you tap twice on it, a line of bright LED dots lights up - up to five, indicating your progress so far that day toward whatever activity goal you set for yourself (say, 10,000 steps). Each dot indicates 20 percent toward your goal; achieving 100 percent earns you a vibrating, five-light flashing rubber-band celebration.

The third improvement is that you can remove the Flex’s heart - a tiny, shiny black capsule that’s held snugly in a pocket within the rubber. (In fact, you must remove it to charge it. The capsule snaps into a USB charging cord for that purpose. One charge lasts about five days.)

The beauty of this design is that you can snap that capsule into other Flex bands. You’re not stuck with the look of the original. Once you have bought the original gray-band or black-band Flex kit, you can buy a $30 set containing teal, orange and navy blue bands. And Fitbit may offer other styles in time.

Which is lucky, because the Flex has one sizable disadvantage: it’s hard to put on. The clasp - two metal prongs that snap into cutouts in the rubber - holds fast, but it’s fussy to connect.

Fortunately, you don’t have to take it off much. You wear it sleeping, exercising, eating, swimming, showering. At any time, you can open the phone app to read the latest stats for the day: steps, distance, calories burned and so on.

The Flex even comes with a tiny plug, about the size of half your thumbnail, that plugs into a USB jack on your Mac or PC. Any time you walk by the computer, the band transmits the latest measurements. In other words, you can use the Flex band even if you don’t own a trendy phone.

In all of these ways, the Flex hardware is more convenient, more flexible and less expensive (by 30 percent) than its archrival, the Up band. Alas, the software isn’t nearly as impressive.

It can show you daily, weekly, monthly or yearly graphs of your steps taken or calories burned, but can’t plot that data with other factors. For example, the Up software can plot hours of sleep against physical activity, so you can see how exercise affects your sleep.

Both bands can track your sleep, but neither can sense automatically that you’re going to bed. You have to tap the Flex band six times rapidly, until two LED dots fade out, to tell it that you’re retiring.

Once it knows you’re in bed, the Flex software tracks the number of hours you sleep, when you’re “restless” (not really asleep but not fully awake), and how many times you wake during the night. But its sleep graph doesn’t distinguish light sleep from deep sleep, as the Up band does. You miss out on a fascinating window into yourself.

The Flex’s alarm feature can vibrate on your wrist to wake you, but doesn’t offer a power nap feature like the Up’s. The Up band can also wake you at your lightest phase of sleep within 30 minutes of your set alarm time, in an effort to reduce grogginess. The Fitbit doesn’t do that, either. Nor can it replicate the Up’s trick of vibrating, after every long stretch of inactivity, to remind you that you’re turning into a gigantic slug.
Neither band can track what you eat automatically - that would be not so much technology as witchcraft. Both require you to record your edibles manually. The Up makes that job easier and more visual, although few people bother.

On the other hand, the Flex lets you record your daily weight, which you could argue is an important health indicator. In fact, if you buy Fitbit’s $130 wireless Aria bathroom scale, your phone can track that figure automatically, too.

Both bands let you share your daily measurements with friends, relatives or blood rivals who own the same band. It’s amazing how well public humiliation works as a motivational force.

Fitbit, the company, acknowledges that its software offers fewer features than Up’s - but points out that its app is therefore less cluttered and easier to navigate.

In the end, though, comparing features is splitting hairs. All of these gadgets perform the Big Feature well: making you constantly aware of your inactivity and lack of sleep - and motivating you, gently and engagingly, to pilot your life onto a healthier track.