t's common to see your glucose dip while you sleep, and there are several reasons why. Understanding the causes can help you interpret your overnight data more confidently.
1. Fasting state
While you sleep, you're in your longest fasting window of the day. Without food coming in, your body draws on stored glucose for energy, and blood sugar naturally trends lower. This is completely normal and expected.
2. Compression lows (the most common culprit)
If you see a sharp, sudden drop in glucose while sleeping, it's very likely a compression low. This happens when you sleep on or near your sensor, putting pressure on the tissue around it. The pressure reduces blood flow and the amount of interstitial fluid the sensor can read, causing it to report artificially low glucose values. These are not true lows; your actual blood sugar is fine. You'll usually see the reading bounce back once the pressure is relieved.
3. Daytime variability carrying into the night
Days with more glucose swings (spikes and dips) can lead to continued variability overnight. If your body spent the day responding to sharp glucose fluctuations, that metabolic activity can carry through into the night.
4. Late evening meals and alcohol
Late meals: Eating close to bedtime can cause a glucose spike followed by a reactive dip as your body processes the food while you sleep
Alcohol: Alcohol inhibits your liver's ability to release stored glucose, which can lead to lower overnight readings, sometimes significantly so
5. Hormonal rhythms
Your body's counter-regulatory hormones (like cortisol and growth hormone) follow a circadian rhythm. These hormones are less active during the late sleep phase (roughly 3AM to 7AM), which means your body has a weaker response to falling glucose during this window. This is also why the Dawn Phenomenon (a rise in glucose in the early morning hours) occurs as these hormones kick back in before waking.
6. Other contributing factors
Several other factors can influence overnight glucose:
Exercise earlier in the day: Physical activity increases insulin sensitivity for hours afterward, which can lead to lower glucose overnight
Medications or supplements: Some can affect glucose metabolism during sleep
Illness or inflammation: Your body's immune response can alter glucose patterns
Microbiome activity: Gut bacteria continue to process food overnight, which can influence glucose levels
When should you be concerned?
Occasional dips below 70 mg/dL overnight are common on CGM, especially on days where your overall glucose averaged lower and showed less variability. However, if you're consistently seeing very low readings, it's worth checking whether:
You're sleeping on your sensor (try repositioning)
Your sensor may be loose, errored, or nearing end of life
There's a pattern worth discussing with your provider
Sensor issues
If your sensor has loosened, errored, or stopped working correctly, that can show up as inaccurate overnight readings. If you suspect a sensor issue, reach out to our team.
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