Sensor seems accurate during the day but reading low at night

I'll start this by saying that I'm on my very first Dexcom sensor ever. I have been wearing the G4 for 30 days and it is often spot-on during the day, but then reads lower than my meter at night. This is particularly annoying because if my meter tells me 90 at night, my Dex is beeping 55 and making me lose a lot of sleep!

At what point do you say that the sensor is no longer accurate enough? I simply don't have anything to compare it to yet so don't want to yank it for a few readings off by 30 points, especially when it is still accurate during the day. But the lack of sleep is starting to get to me. I have had far too many false low alarms in the night (including the last two nights with a straight red line from 1am to 8am).

Thanks for sharing your experience!

The same thing has happened to me. Once I have a few nights like that, I change the sensor, especially if I am well beyond the suggested 7-day wearing period. My sleep is important!

Thanks for that quick response. Always nice to know when someone else has "been there". I know I can't complain about the length of time i've been going on this sensor! But I don't have insurance coverage for CGM so I'm definitely trying to push the limits. Since it's almost bedtime, I'm going to give it one more night. Then I'll change it tomorrow if i have a repeat false hypo night.

I extend to two weeks, but that’s it. My readings are good, usually. Your experience going out to 30 days does not surprise me. I would venture to guess that at your 30 days you are really pushing the sensor’s effective life (even if your sensor is still ‘working’ ). And 30 days is by no means the longest that sensors last (a Dexcom rep told me they tested at UCSB and reached 88 days on a single sensor!).

So, I just think your sensor is giving it up. Time for a change.

Regardless of how you handle this one, the sensors do have use by dates that are pretty firm. There’s been threads on this subject here before.


I often experience lower than accurate readings over night. I have yet to determine the cause, though it seems to happen more frequently when I don’t drink a large glass of water before bed. I’m thinking I get dehydrated a bit overnight and that throws off the readings. In my case, it had nothing to do with the length of time that I’ve worn the sensor.

I've noticed this too (although mine doesn't go below 55), but because of this I set my Low Alert to 60 at night (whereas I have it set to 75 during the day).

This works great for me, except when I forget to reset it to 75 in the morning and hit 60 without any warning during the day :-(

If you're reading this Dexcom, for the G5 it would be great to be able to set 2 different alerts depending on the time of day! :-)

It happens because blood flow slows down as we sleep and the sensor isn't able to accurately guess our actual bg. I turn off my low alert at night (or at least lower it).

I have to agree with what someone else said about hydration. I find that when I don't have adequate fluids I tend to see it false both high and low. Also sometimes if I sleep with the sensor on the mattress that seems to mess with it at times.
But at 14 days I am usually replacing my sensors as most of the time they fail on me around day 16-17 if I wait. I rather it not die on me during an important activity or work.

Thanks, all of you, for these tips. I did lower my alert level overnight and that helped. I'll try drinking more water before bed too, but then that gets me up in the night for other reasons!
This is a slightly different topic, but I hesitate to change the sensor that seems so accurate (other than the nighttime problem). As I said above, I don't have insurance coverage for these so any sensors I buy, I'm paying full price. I had good tape advice from someone who regularly gets more than a month from his sensors and so far the itching has been minimal. I certainly didn't go into this expecting it to last a month, but so far, i'm just going with the flow to get an idea of how much I'm willing to invest in this technology over the course of a year.

I have to agree with you.
If it stays on and is accurate as ever (without any ??? issues) I would keep it on, especially if you are paying full price.

The only times I've taken them off earlier (when everything is going well) is because I am afraid the insertion spot might need to heal, but once I've taken it off it's actually minimal and the site is completely healed a few days later.

(Of course this is just my experience and how my body reacts to the G4!)

I had my first ??? after about 26 days and I think it was because I lay on the sensor for about 10 minutes while putting the kids to bed. The ??? only lasted about 45 min and then was as accurate as ever. I'm fighting sticky-ness of the tape more than anything to keep it on but so far so good. I am actually amazed at how little it has itched since I have fairly sensitive skin.

I find it's important to "train" a new sensor. If you enter bG readings across the entire spectrum of your bGs, the sensor can figure out the correct bG.

Re: nighttime in particular, I've noticed that my readings are off if I've been sleeping in a position that puts pressure on the sensor. I determine my sensor location in part based on where it won't get pressed on.

I have had this issue too at night. For me it was caused by putting direct pressure on the sensor, so if you lay directly on the sensor, the readings will take a dip every time. I am not sure if this is caused by reduced blood flow or if the wire is bending a little bit from the extra pressure. The readings will be tracking along very flat and then take an unrealistic drop, then when you stop laying on the sensor the readings will come back to normal after an hour or so. I make an effort to try and go to sleep in a way that I am not laying on the sensor, but often times I will toss around in my sleep and get a false low. After I figured out what was causing the issue I was able to minimize the frequency that it occurs and understand when the readings are incorrect and just ignore the occasional false reading.

I have definitely been actively trying to stay off it during the night but cannot guarantee that I don't roll onto that side while sleeping. From the trends i've had, the decreased blood flow generally while sleeping seems to me more likely for my situation, since I haven't had major unrealistic drops, just slowly dropping and if the low is confirmed on my meter, dexcom not responding to glucose the way it does during the day. (though I remember those unrealistic drops from when I wore the first generation (blind) medtronic CGMS in about 2001, where I'd drop off suddenly in the middle of the night and stay at <40 for a few hours then pop back up again.) I don't mind "ignoring the occasional false reading", it's just that it's hard to ignore the Beep beep beep beeping all night long! The lack of sleep isn't helping the overall bg control!

Sensor placement change might help...somewhere which you do not sleep on top of or somewhere with better blood flow. My dex consistently reads on average 17 points low, so one solution might be to enter in higher numbers when you test.

I have the older Seven model, and insurance. I've been changing every 7 days & noticed the same problem. Very annoying. i keep my Low Alert at 60 and put up with it, since I live alone & might wake up at 42 some nights without any alert at all. (Yes, hydration may help, but then I'm up at night for sure!)