(c) someone talented on the interwebz
I know MiniMed CGMs are known for their quirks and inaccuracies. However, I’ve gotten up one too many times in the middle of the night to find out that my sugars aren’t what my CGM says they are.
Last night was no exception. By the 4th wake up call and checks to discover that my sugars were safely in the 80s, I gave up listening to my CGM. I even restarted it because it said I had a double arrows down to 56 when I was actually 102. I just wanted sleep, damnit.
At around 5:30 am, I got another low alarm. This time for about a 74 mg/dl. Fine. Whatever… I’m sure I’m okay and 70 is actually an ideal fasting blood sugar for pre-pregnancy prep. So I continued to sleep until my alarm went off at 6:15. But man, I was hungry.
Woke up to a blood sugar of 56 mg/dl.
Oh… I see how it is. <sigh> This is what I was afraid of.
See Jen Dance 
Ugh, I know exactly how you feel except mine is usually false for a high. I have had too many nights where it has gone off multiple times, I test and no I’m actually not as high as it says…grrr…although to not be too negative, it has alerted me a lot of times too to a high that has helped me to wake up with a good blood sugar-like last night, it woke me up at 250, it was actually correct, I bolused and woke up at 100 🙂
My Dexcom acts the same with a fresh sensor (day 1 and sometimes 2) and then with a sensor on it’s last legs of use (sometimes day 15, sometimes as late as day 21.) I use the fingerstick meter ANY time I question the CGMs validity, and if it’s acting truly wonky overnight, I just turn the thing off and sleep. I survived for years without it, so one night without it during the time it’s acclimating isn’t a deal breaker, since I LOVE it for the other 98% of the time it’s on 🙂
Nothing is all that reliable when sleeping, I find. Your body isn’t moving, and the blood – and the glucose it contains – is slowly meandering through the vessels like a lazy stream rather than a raging river. I’d bet this leads to different BG concentrations at different parts of the body, an at a particular moment, the CGM could be leading the fingerstick in time, not trailing it. It all depends on where your sensor is, where your arms are, and how you sleep (my unproven theory).
It’s why I try hard to resist the urge to calibrate as soon as I wake up. I try to start my day and get the blood flowing first, just to make sure everything is accurate and consistency.
My instinct is to calibrate upon waking, just because there isn’t insulin in my system and I was told repeatedly by my CDE and Medtronic rep to not calibrate with IOB. But – your theory makes sense too. Maybe I’ll have to experiment.
My low alarm kept going off last night, and I kept silencing it. Finally my husband asked me if I needed juice, and I said, “No. It’s wrong.”
I finally got up and checked my blood: 115.