The 3am alarm that does not quite make sense
You are asleep, the CGM alarm sounds and the graph appears to have fallen off a cliff. You wake up expecting familiar hypo symptoms—but you feel fine. A few minutes after rolling over, the reading starts climbing again.
That may have been a compression low: a falsely low sensor reading caused by pressure around the sensor site. But “may” is the important word. A genuine hypo can also happen during sleep, and it is never safe to dismiss a low alarm automatically.
What causes a compression low?
A CGM measures glucose in interstitial fluid beneath the skin rather than directly in the blood. Sustained pressure on or around the sensor can reduce local fluid circulation and interfere with the reading.
This most commonly happens when sleeping directly on the sensor, although tight clothing, leaning against a chair or trapping the sensor under the body can also contribute.
The result may be a reading that drops suddenly even though blood glucose has not fallen in the same way.
Clues that make me suspect compression
No single clue proves a reading is false, but a compression low often has a recognisable pattern:
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The graph drops much faster than expected
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The low begins while lying directly on the sensor
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I have no symptoms, although symptom absence never rules out a hypo
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Recent food, insulin and activity do not explain the fall
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The reading starts recovering after pressure is removed
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A blood glucose meter gives a substantially different result
A real hypo can also fall quickly and may occur without obvious symptoms. That is why context can raise suspicion but cannot provide certainty.
What I do when the alarm sounds
First, I take the alarm seriously. If I have clear hypo symptoms, cannot safely check, or feel there is any immediate danger, I follow my personal hypo treatment plan. That being said, I always have a testing kit in my bag, 1 in the car, and 1 in the bedside table.
If the reading seems unusual and I can check safely, I remove pressure from the sensor and use a blood glucose meter. Dexcom advises using a meter when readings do not match symptoms or expectations and making treatment decisions from the meter result in that situation.
I do not simply watch and wait through a suspected genuine hypo. If the meter confirms that I am low, I treat it according to the plan agreed with my healthcare team and recheck as instructed.
If the meter is in range and the CGM recovers after I change position, compression becomes a much more likely explanation.
Why CGM and meter readings may differ anyway
Even when a sensor is working normally, CGM and finger-prick readings will not always be identical. They measure glucose in different places, and interstitial glucose can lag behind blood glucose when levels are changing rapidly.
Meter accuracy, hand contamination, sensor age and the natural delay between blood and interstitial fluid can all affect the comparison. The goal is not necessarily to see two perfectly matching numbers; it is to decide whether the CGM reading makes enough sense to use safely.
Can compression lows be prevented?
Not completely, especially for anyone who moves frequently during sleep. A few practical changes may reduce them:
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Consider usual sleeping positions when choosing an approved sensor site
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Avoid placing the sensor where waistbands or tight clothing press repeatedly
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Use pillows to reduce rolling onto the sensor if comfortable
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Keep a blood glucose meter and hypo treatment by the bed
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Notice whether one site repeatedly causes overnight false alarms
Always use sensor sites approved for your device and age group, and ask your diabetes team if you need help finding a suitable location.
When to contact Dexcom, CGM manufacturer or a healthcare professional
Seek help if readings repeatedly do not match symptoms or meter checks, the sensor produces unexplained gaps or errors, the site is painful or inflamed, or you are unsure whether the system is safe to use.
Repeated night-time hypoglycaemia, reduced hypo awareness or any severe hypo also deserves discussion with a diabetes professional. The answer should not be to assume future alarms are false.
A compression low is annoying, noisy and capable of ruining a good night’s sleep. A genuine hypo is more important. When in doubt, check—and put safety before the shape of the graph.