My niece is on Day 8 (we restarted Dex today, what a dream come true after MM’s restarts) of her very first Dexcom sensor. Rep inserted it, along with her Mom, so this one is a good one. Yes, we have had ??? almost every night around 3am thru 5am. She is having a growth spurt, BGs are rising extremely rapidly, necessitating double her basal. After three hours of this, Dex kind of throws in the towel. We left Dex alone and when her BGs stabilized around 5 or 6am, we got the readings and then calibrated. During times of steep rises and steep drops, it can be off 30 percent, at least for us. This was true with the Minimed sensor as well, only we had so much trouble calibrating. BG really needs to be stable calibrating for MM sensors. With Dex, it seems it needs to be semi-stable. We can calibrate at a higher BS level. I’m sure others will chip in and tell you how to get Dex to read very accurately, as you will need it for mountain biking. I don’t expect the cgms to be extremely accurate, just reasonably so. After dinner when she is spiking, I can still get the trend arrows and that helps immensely. Overnight, I have been getting the ???. Will be even more aggressive with the basal overnight tonight, keep checking, and hope we don’t lose Dex readings. I appreciate the pain-free insertions, and the readings have been more accurate than the Minimeds so far. I really love how we don’t have to take the sensor off every three days, put it on the receiver, and put it back on to restart. Dex sensor stays on the body for seven whole days, and you leave it on the body when you restart. It seems to get even more accurate as time wears on. At some point, it will lose readings, but readings have been great after restart so far.