Dexcom nighttime accuracy issues

My Dexcom G4 has recently been showing values 20-30 points below actual, but only in the middle of the night. Daytime readings are pretty accurate and usually vary only 5 points from fingersticks, but the nighttime readings are driving me nuts. Last night, for example, I was woken up by the alarm and a reading showing 55. Concerning to say the least, but I didn't feel hypo. I dragged myself into the bathroom all bleary eyed and did a test with my meter: 84. Of course, I couldn't fall back asleep properly and have started my week in a fog...

This has now happened several times in the last couple of weeks. My insertion sites have remained consistent for the last several months on the back of my arms and my nighttime readings have been accurate until recently.

Is this happening to anyone else? Any ideas what might be causing this?



Are you sleeping on your arm? I have found issues if I sleep on the sensor. I always assumed the pressure on the sensor must be changing the normal circulation.

If you are laying on the sensor when you sleep you can compress the tissue and restrict circulation leading to false lows. I tend to roll over off of it and wait for it to alarm a second time before testing. I have my kit on the headboard so no need to get out of bed.

One other possibility is now with the warmer weather you are not as well hydrated at the end of the day. This will effect numbers as well as it tends to slow circulation. An extra glass of water before bed can help in that case. Sometimes it's a combination of both issues.

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Yes, but there is that nasty side-effect of having to get up and pee in the middle of the night sometimes...

With experience you learn how LARGE a beverage to have before bed. And beer does not count as it will make you pee in the wee hours.

Great insights. Thank you! I'll try a large glass of water tonight before bed and will be sure to see which arm I'm lying on if the alarm goes off.

I’ve also experienced similar low alarms (by about 20 to 30 points compared to fingersticks) on the Dexcom during the nighttime. On some occasions, sleeping on the sensor seemed to have triggered the low alarm. On other occasions, it’s unclear what caused the inaccurate low readings. By the next morning, the numbers are about 30 points apart. I try to drink a reasonable amount of water before bedtime so that I don’t have to wake up in the middle of the night. The numbers during the daytime are fairly accurate.

Any other thoughts and feedback?

if anything like other sensor brands, putting pressure on the sensor area will push interstitial fluid away from the sensor, causing lower readings. with my Enlites, pressure during sleep time, against the sensor can drop the readings more than 70 points. Upon getting out of bed, typically the numbers come back up rather quickly. I desperately wish that all sensor systems could key off of blood instead, for immediately accurate results. sigh.

Some Enlites I’ve worn have been able to maintain accurate numbers throughout the night.

Upon getting out of bed, the numbers on my dexcom don’t come back. sigh…

I have that problem with some of my Enlites, too. At some point, I call MM for free replacements when the electrical signal (ISIG) fails to reach appropriate levels for the glucose levels.

When your heart rate slows down your sensor falls out of calibration and when you get up it gets back in sink.
The sensor is really a oxygen sensor, When I was in the hospital my oxygen level would drop at night and set off the alarms
so after a few nights the nurses figured out it was better for them if they gave me a little oxygen, during the day my
saturation was 97-98 % but at night it would drop below 90% when I was really sleeping good. My CGM sensor
would also drop off and flat line…

Who told you that oxygen levels are the cause of low CGM values?

And when it comes to Enlites, unless there has been a huge change in the chemistry from the previous MM sensor, oxygen doesn’t enter into the readings AT ALL, so I take it you are referring only to Dexcom sensors… I don’t think the average person has their O2 level drop to 90% when sleeping anyway…perhaps YOURS did, but that’s atypical of a diabetic using a CGM in their daily life. Interstitial fluid, being shoved away from a sensor is the big factor in low readings.

here is a factoid for sleep apnea