“The UP wristband is meant to be worn 24 hours a day. When you’re awake, its accelerometer monitors your movement–whether you’re running, walking, or climbing stairs–and then sends that data to the app, which shows how many calories you’ve burned. When you’re asleep, the UP monitors your sleep stage, by tracking subtle fluttering wrist movements (a natural occurrence during REM sleep, which is similar to eyelid flutter). When its time to wake up, the wristband vibrates slightly, and times its alarm to the best phase of your sleep cycle. And finally, the UP smartphone app allows you to take pictures of your food and log your meals.
The cleverest features, however, are a bit more subtle. The UP isn’t meant to be a passive health-monitoring device–if so, it would be hard to see how people would keep using it, given how often, for example, diets fail. Instead, it’s meant to constantly nudge you into better behavior. For example, you can set the wristband to vibrate when you’ve been sedentary for too long–a reminder to keep moving around. There are also challenges you can take on, such as running or walking a certain distance each day, or biking to work three times a week. Users can track their progress as they go along, and they can choose challenges created by others (including professional trainers and public-health experts).”
(see also this Wired article)