What a compression low is
A compression low is a false low reading caused by pressure on the sensor. It happens most often at night, when you lie on the arm or the side where the sensor sits. The pressure changes what the sensor can measure, and the line on your phone dips even though the glucose in your blood hasn’t moved much.
It’s one of the most common surprises for people who wear a CGM, and it can happen with any sensor brand, because it comes from how sensors measure glucose under the skin.
What it looks like on your line
A compression low has a recognizable shape:
- A sudden drop that doesn’t follow the trend before it: the line was flat or drifting gently, then falls steeply.
- A flat or ragged bottom while the pressure lasts.
- A fast rebound, often back to where the line was, when you change position.
- A typical time: during sleep, and often on nights when you sleep on the sensor side.
A real drop in glucose usually builds more gradually and doesn’t snap back the moment you roll over. The table below sums up the differences, but treat it as a guide, not a test.
| Often a compression low | More like a real drop | |
|---|---|---|
| Start | Sudden, out of step with the trend | Gradual, follows a trend |
| When | During sleep, lying on the sensor | Any time, including after exercise |
| Recovery | Quick rebound when you move | Stays low or recovers slowly |
| How you feel | Fine, or asleep | May match symptoms |
Why it happens
A CGM sensor sits in the fluid between your cells, just under the skin, and measures the glucose in that fluid. Its readings depend on a steady exchange of fluid and glucose around its tiny filament.
When you lie on the sensor, the pressure squeezes the tissue around it and slows that exchange. Locally, less glucose reaches the filament, so the sensor reports a lower value than the one in your blood. Take the pressure away and the fluid flows again: the reading climbs back, sometimes within minutes.
Compression low or real low?
The shape is a strong hint, but it isn’t proof: a real low can also happen at night, for example after a long or hard workout the day before (see glucose during exercise).
If a reading doesn’t match how you feel, sensor makers advise confirming it with a fingerstick before you act on it. Follow your sensor’s instructions and your healthcare team’s advice.
Fewer compression lows
People who get frequent compression lows often find these habits help:
- Choose the side you don’t sleep on, within the wear sites your sensor’s instructions approve. If you always sleep on your left side, the right arm is the obvious candidate.
- Keep weight off the sensor: a pillow behind your back or between your arms can stop you rolling onto it.
- Notice your patterns. If the dips cluster on certain nights or positions, that’s useful to know, and to mention to your healthcare team.
What it does to your stats
Your sensor has no way to flag a reading as “pressure”, so a compression low counts like any other reading. A long one can lower the average for that night, cut its time in range and even register as a low in a weekly summary.
Ando calculates its numbers from the readings your sensor sends, unchanged. It doesn’t hide or edit readings, because it can’t know which ones came from pressure. If one night’s numbers look odd, open the night and look at the shape of the line.
Seeing your nights in Ando
Ando puts your overnight glucose next to your sleep from Oura, WHOOP, Garmin, Apple Watch or Fitbit: deep, light and REM sleep, time awake and the night’s time in range, on one screen. Seeing a sudden dip next to the rest of the night makes it easier to read.
More on overnight shapes in how to read your overnight line, and on how Ando shows your nights on the sleep page.
Questions people ask
What is a compression low on a CGM?
It is a false low reading caused by pressure on the sensor, usually when you lie on it during the night. The pressure slows the flow of fluid around the sensor’s filament, so it reports less glucose than there really is. The reading recovers once the pressure is gone.
Do compression lows happen with Dexcom and FreeStyle Libre?
Yes. Compression lows are a known effect of how CGM sensors measure glucose under the skin, so they can happen with any brand: Dexcom, FreeStyle Libre, Medtronic, Medtrum and others.
How long does a compression low last?
Usually as long as the pressure lasts. Many are short, from a few minutes to under an hour, and end with a quick rebound when you change position. A low that persists after you move is a different situation.
How can I tell a compression low from a real low?
A compression low tends to be a sudden drop that does not fit the trend before it, often at night, with a fast rebound when you roll over. But the shape alone is not proof. If a reading does not match how you feel, sensor makers advise checking with a fingerstick before acting.
Does Ando remove compression lows from my data?
No. Ando shows the readings your sensor sends, as they are, and calculates your stats from them. It does not guess which readings came from pressure. Seeing the night next to your sleep stages can help you put a sudden dip in context.