It was one of the features I anticipated the most alongside advanced fitness and bio-metric data collection. As rumours suggest they were looking into integrating blood pressure, o2 saturation, hydration levels etc. now the watch doesn't even have a sleep tracking feature. Most probably there will be one via a 3rd party app but my expectations were so high when the drawings of the "Health App" came from the rumour mill of iOS 8. There always will be a second watch and iOS 9 but I'm let down they didn't lean more into fitness/activity stuff. I would love to see more than what Jawbone is offering with UP3. I don't want to care even more about notifications, they zone me out from social situations enough as is. I respect that it might fit some people(especially very, very busy ones apparently) but right now it is a definite no buy for me, a very expensive iPhone accessory, for this gen. at least. And I don't expect it to be huge in sales besides the Apple fan community. Maybe in the future like the original iPhone, but not now. edit: we don't even know if the watch is capable of tracking heart rate all the time without smashing battery life.