How Do I Find Overnight Glucose Dips in a Month of CGM Data
Export your CGM file into the free HealthOS report and read the overnight section for short sharp dips that recover fast, which often mean compression lows.
For a month of CGM data, the fastest way to find overnight dips is to export the sensor file and run it through the free HealthOS CGM report. In the report's overnight section, look for short sharp dips that recover quickly; the report often marks these as possible compression lows. Overnight dips in a healthy person wearing a CGM are often pressure artifacts from lying on the sensor, not real blood glucose events.
How do I find overnight glucose dips in a month of CGM data
Export your sensor's data file first. The report accepts LibreView .csv, Dexcom Clarity and Stelo .csv, Lingo export, SiBionics .xlsx, Apple Health export.zip, or any CSV with timestamps and glucose readings. Drop the file on the report page. The file is read in your browser and never uploaded.
When the report opens, go to the overnight section. Look for dips shaped like a narrow V. The reading starts from a stable baseline, for example 90 to 120 mg/dL, drops fast into an illustrative low range such as 55 to 75 mg/dL, stays there one or two readings, then snaps back up. With a short sensor interval, the dip may be brief; with a longer sensor interval, you may only see one low reading before it corrects.
Scan the whole month, not one night. A compression low can show up at random times on random nights. A real overnight dip may recur at a similar time across several nights, but that pattern alone is not enough to confirm it.
What a compression low looks like
The shape on the graph is the tell.
Illustrative shapes and timing only. A compression low drops fast and recovers quickly; a real dip may be flatter and slower.
Compression lows happen because a CGM reads glucose in the interstitial fluid just under the skin, not glucose in blood. When you lie on the sensor, your body weight squeezes the tissue around the filament. That pressure pushes fluid away from the sensor and temporarily reduces local perfusion, so the sensor reports a low reading even though your circulating blood glucose has not changed. Roll over and the pressure releases. Fluid returns and the reading snaps back.
A short sharp overnight dip that recovers as soon as you move is usually the sensor reporting pressure, not your blood glucose.
Compression low or a real overnight dip?
The practical question is whether the dip is something you need to act on.
| Feature | Compression low | Real overnight drop |
|---|---|---|
| Onset | Sudden, over one or two readings | Slower, over several readings |
| Bottom | Brief, one or two readings | Prolonged until the body corrects or carbs are eaten |
| Recovery | Immediate on position change | Takes time, may overshoot |
| Pattern across a month | Random nights and times | May repeat at a similar time |
| Finger-prick check | May be normal | Usually close to sensor reading |
Scroll the table sideways for the rest →
Illustrative. Patterns in CGM data. A finger-prick check can help separate the two when a low looks real.
People commonly misread compression lows as nocturnal hypoglycaemia. The mistake is treating a sensor reading as a blood glucose reading. CGM sensors read interstitial fluid, and local pressure can temporarily decouple the two. As an illustrative example, a finger-prick check right after a sharp overnight low may show 80 to 100 mg/dL while the sensor still shows a low reading such as 60 mg/dL. That gap is the artifact.
What should I do with an overnight dip?
- Note which side you sleep on and whether the sensor is on that arm. Dips that line up with lying on the sensor are often compression lows.
- If a low repeats at a similar time, is prolonged rather than brief, happens on different sensor sites, or wakes you with sweating, shaking or a racing heart, check with a finger-prick meter.
- If you are unsure, check with a finger-prick meter and talk to a doctor.
- If you take insulin or a sulfonylurea, a sensor low should not be dismissed without checking. The report's overnight flag is a pattern marker, not a diagnosis.
What the free report cannot tell you
It cannot know which side you were sleeping on, whether there was pressure on the sensor, or whether you had symptoms. It marks short, sharp dips that recover fast as possible compression lows because the shape is very specific. It cannot confirm a real low or rule one out. It also cannot say why a real overnight dip happened if you have one.
Some non-diabetic CGM users may rarely see true low glucose, but this varies. When a true low does happen, the trace often looks flatter and slower than a compression dip. The sensor cannot distinguish local pressure from systemic low glucose. A finger-prick meter can. The report is a wellness tool, not a medical device.
If you have a month of data, run it through the free CGM report, drop your LibreView, Dexcom Clarity, Stelo, Lingo or SiBionics export and see your AGP, time in range and biggest rises in the browser with no account. If you do not have a sensor yet, HealthOS Sensor gives you 30 days of continuous glucose with the app for $70, no membership at https://healthosx.com/glucose/sensor.
Written by
HealthOS Research