Dexcom Night Time Low Readings

I’ve experienced a recurring phenomenon when using my Dexcom 7+ at night. During the day the BG readings are very accurate but at night the readings typically run 20-40 points below my actual (i.e. it reads 60 when I’m really 90). As a result I get many false alarms re hypo events at night. I’ve discussed this with Dexcom but they don’t have any answers. I was wondering whether any others have experienced this and have any ideas for mitigation. I’m a very happy Dexcom user but getting the night time thing straight would be great. Thanks for any advice or thoughts. Mike.

My son has been using DexCom 7+ for just over a month. We originally had trouble at night but the last couple of sensors have been rather accurate (knock wood). One of our issues was different than what you describe. We had “out of range” or “???” for most of the night in addition to inaccurate readings. Once I moved the receiver from his nightstand to his bed this has corrected (for the most part). I’m just wondering where you keep it at night. Keeping it in bed was suggested here by another user and it seems to have helped not only me, but other users as well

Good luck! Those false alarms are annoying! I hope they subside.


Thanks for your reply. Luckily I do not experience any ??? during the night but the false lows are similarly frustrating. I leave the receiver under the covers about 2 ft away from me and find that this works well. If I place it on the bedstand my readings get interupted - but not with ???, just a lost signal with the transmitter symbol. Glad to know that your Dexcom is working well for your son - it certainly is an invaluable tool. Good luck to you too. Mike.

Oh well. So sorry I couldn’t help. Glad to know that keeping the receiver in the bed seems to consistently improve readings though. Hopefully someone else here will come up with something! Lo

This is something that I have noticed as well. I keep mine on the bed next to me (a silicone skin keeps it from disappearing), but noticed that when I’m sleeping on my stomach I will get some incorrect readings. I have also noticed that mine can run 20-45 points below or above my glucose meter. I’ve also noticed great fluctuations in my glucose meter as well, but I’m sure that’s another story.

Sounds like I’m not the only one experiencing this. I have not tried sensor locations other than in the abdomen, as recommended by Dexcom, but I wonder if other sites might be less prone to this discrepancy. Someone suggested to me that the interstitial fluid near the sensor site might be pooling at night when one is not moving as much. The latter could cause readings to be lower at the sensor site than elswehere in one’s body. Not sure whether the science of this explanation holds up. Thanks for your reply.

I have gone thru this each time I have to train a new sensor. This is a calibration problem. After the two initial startup calibrations, I follow Rick27’s advice: Enter calibrations when you’re flatlining, and give it numbers at least 30 mg/dL apart. Try to get one in the middle of your range, one near the high alarm, one around 80. Do not enter calibration at bG below 75. And Never enter calibration during a rising BG. Once I started following these rules, I found that I was going thru this annoying low alarm thing only on the first night, and by the second night it was on track.

Thanks for the helplful advice - I will find Rick27’s post and give it a try. It still seems mysterious to me that daytime numbers would be unaffected and accurate while nighttime is a different matter. I would have thought that a mis-calibration would pertain to all time periods but I must be missing something.

Hi Mike. I think that etta’s post (below) is referring to some general calibration advice which I’ve offered elsewhere. (Do give them a read, and remember that YMMV.) They don’t deal with the specific issue of bad accuracy during sleep (with good accuracy at other times).

The “science of this explanation” DOES hold up for many of us-- especially if you end up sleeping for lengthy periods of time with your site lower than your heart. It’s far, far worse if you actually sleep with pressure right on the site (belly against the mattress). ISF does get “stale”, with inadequate refreshment from capillary blood supply, under these conditions. Locally, cells continue to draw glucose from ISF, leading to excessively low readings on Dexcom.

Locally, these cells really are experiencing the “low” which Dexcom shows, but cells within critical organs with better blood supply (brain, heart, liver, and etc.) are not having any problem at all. A bG fingerstick reading may or may not have similar accuracy problems, it’s all a question of glucose consumption versus glucose supply.

To reduce this effect, I sleep on my side, and choose which side to sleep on according to the Sensor location-- whichever love handle or outer/upper butt cheek has the Sensor, that’s the side which stays high up. If I’m unsure of a reading and crawl out of bed to check bG, I’m sure to “cartwheel” the arm where I’m going to do the test, and clench unclench the fingers of the hand I’ve chosen, then wait about 10 extra seconds before pulling out the test strip and starting the fingerpoke ritual.

Never calibrate from bG readings taken during sleep.

I read on CWD that laying on a sensor can cause bg to drop. Like Etta I only notice symptoms during the first night. What I see on the CGM are ups-and-downs that could coincide with me turning from one side to the other. During all other nights my bg is sloping only slightly. I reason that putting pressure on a fresh wound causes my body to react. This is the quote from CWD:

“It would be wonderful… only thing is they (or we) need to come up with something to prevent pressure on the sensor while sleeping. If you lay on a sensor, BG can drop to 40, and if insulin was cut off, the next thing you know could be DKA. But if the sensor had some kind of pad or casing around it to prevent pressure-induced false lows, this might actually work!”

Hi Rick: Many thanks for the helpful information - it makes total sense to me now. Since I always sleep on my back and don’t move at all during the night (my wife has confirmed this oddity), the sensor is clearly always lower than my heart. I’ll have to figure out how to sleep on my side with the sensor facing upwards or else see if a forehead placement would work (just kidding…). Perhaps even an upper arm/shoulder sensor placement would be marginally higher than a side placement in the event I am unable to avoid sleeping on my back. I’ve never put the sensor anywhere but my side but it would seem to be worth a try. I really appreciate knowing the science behind this phenomenom - we are very fortunate having someone like you who is willing/able to explain this to us. I’ve read some of your other posts and they are similarly extremely helpful. Thanks again. Mike.

Thanks for your reply and insight. I’m planning on utilizing all the helpful advice and information I’ve received today.

Rickst29, I did draw that info from your general calibration hints. Although this might not be the case for everyone, I definitely have found that the bad accuracy during sleep only happens (for me) on night 1 or 2 of the sensor. I place the sensor in a spot that is not under pressure during the night.

p.s. Thank goodness for Urban Dictionary, so I can look up your abbreviations like YMMV (Your Mileage May Vary, for dummies like me!)

Rick: I found your calibration post before but am now having trouble finding it again. Do you happen to have the rough date for that post. Correct calibration has to be the solution to very many of the Dex 7+ performance issues and I truly appreciate your insights in making these technical issue seem simpler. Thanks. Mike.

I wish we had a better “search” tool, but the one you’re probably looking for is here:

Rick: Many thanks - this is exactly what I was looking for. Yes, I did try the search tool in vain. I really appreciate your posts which are not only fascinating from a technical standpoint but also written with great clarity. Mike.

Rita: Thanks for your reply and your advice. I never sleep on the transmitter and do already use skintac. My daytime readings are great but night time continues to be off by 30-50%. I’ve concluded that the only thing I can do to help the situation is to keep my transmitter higher than my heart (per Rick’s advice) which means sleeping on my side. This forces some more circulation and helps deal with the pooling of glucose-depleted interstitial fluid around the sensor. Dexcom is a great product and I wish you continued good luck with it. Mike.