Could you have been lying on your cgm when you were asleep.? The compression can make readings go way down. In general, some cgms seem to work better than others. I'm wearing one at the moment that is giving readings 0.5 to 1mmol lower than my finger-prick tests. Also, they can take a few days to settle down to accurate readings. When they're working well, they're brilliant, when you get a duff one they're a pain! Over time you get to work out how best to interpret the results and to work out which ones are not behaving well.