I've used this watch for several months so I think I'm starting to see what's up with this situation. When the fitbit app on my phone says that no sleep score could be calculated it has a "learn more" on that same screen that you can click on those words and it takes you to a website that tells you possible reasons. The main thing I have found that solves this MOST of the time is wearing the watch up my arm so that when I bend my wrist, the watch does not move and lose signal. And wearing it tight enough. The web page basically says that if the watch loses contact with heart rate intermittently, and for too many gaps in heart rate signal, the app won't show a score. Because of that, you also generally won't get the 2nd graph in that app that breaks down REM sleep, light sleep, and awake. That is a graph in addition to the basic graph that I find always shows up but is not as in depth. But I've often gotten that 2nd graph but still no sleep score. In my opinion, if I don't even get that graph that also means it lost heart rate signal too much. I also I think to get the score there has to be a long enough period of sleep data in the first place. So you need to get enough sleep on that graph in the first place, in order to register the score. Not only is position on arm important you also have to wear the watch tight. So that website says wear it 2 to 3 finger widths above the wrist bone and that the watchband should feel "secure but not tight". I would say it needs to be tight but not painful lololol. It also states 2 OTHER reasons, if you slept for less than 3 hours or if battery is critically low. But I've had well over 3 hours of data with battery fully charged and still no score. So it must be the heartbeat signal loss and insufficient length of data. But mainly loss of heart rate signal during night. To resolve that, for me it's always the position of the watch and how tight it is. However, this thing is dang sensitive about getting moved. And there are muscles on the top of your arm that run the wrist. So if you put it up too far on arm it'll be right on those muscles and won't work either. Maybe try wearing watch on underside of arm and see what happens. I'm pretty sure that the intermittent loss of heart rate is generally the key. I have tried finding the actual URL for the website but the app doesn't state it. It's just when you click on the "learn more" in the "Why I didn't get a sleep score" in the phone app. If you can find that website, you'll get this same information I'm explaining here. The other cool thing is that they've added the capability to get your average oxygen level while you sleep. Not sure how accurate that is but I think it's as accurate as ones that go on finger. I'm considering testing both on same nights and see what data looks like over several days. Sorry for long explanation but if you buy this expensive watch it's very frustrating if the expense seems wasted.
