A continuous glucose monitor gives you a new reading every few minutes, which means you can quickly end up with thousands of data points and no idea what matters. The key is knowing which numbers to look at first, what the targets are, and how to spot patterns that actually change your daily decisions. Once you learn a few core metrics and a simple review order, CGM data becomes one of the most useful tools available for managing blood sugar.
Start With Time in Range
Time in Range (TIR) is the single most important number on your CGM report. It tells you what percentage of the day your glucose stayed between 70 and 180 mg/dL. For most adults with type 1 or type 2 diabetes, the goal is to spend at least 70% of the day in that window. That translates to roughly 17 out of 24 hours.
The targets for time spent outside that range are just as specific:
- Below 54 mg/dL (serious low): less than 1% of the day, which is about 15 minutes
- 54 to 69 mg/dL (low): less than 4% of the day
- 181 to 250 mg/dL (high): less than 25% of the day
- Above 250 mg/dL (very high): less than 5% of the day
When reviewing your data, always check time below range before anything else. Lows are the most immediate safety concern, and reducing them is the first priority in any CGM review. Once lows are addressed, shift your attention to time above range, then to variability.
What the Trend Arrows Tell You
The real-time number on your CGM screen matters less than the arrow next to it. A reading of 150 mg/dL with a flat arrow is completely different from 150 mg/dL with a double arrow pointing up. The arrows show how fast your glucose is changing over the next 30 minutes, and each arrow type represents a specific rate:
- Flat arrow: glucose is stable (changing fewer than 30 points in 30 minutes)
- Single diagonal arrow (up or down): slowly rising or falling, roughly 30 to 60 points in 30 minutes
- Single straight arrow (up or down): rising or falling 60 to 90 points in 30 minutes
- Double arrows (up or down): rapid change, 90 or more points in 30 minutes
Double arrows demand attention. A double arrow pointing down when you’re at 120 mg/dL means you could be below 30 mg/dL in half an hour. A double arrow pointing up after a meal tells you the spike is steep and fast, which may signal a food that hits your blood sugar harder than expected. Single diagonal arrows are generally fine and reflect normal post-meal rises or gentle overnight drifts.
Reading the Ambulatory Glucose Profile
Most CGM apps generate an Ambulatory Glucose Profile, or AGP, that summarizes 14 days of data on a single 24-hour graph. It looks like a shaded band stretching across the day, and the width of that band is the key visual. A thin band means your glucose followed a similar path day after day. A wide band means your glucose was unpredictable, swinging differently on different days even at the same time.
The AGP typically shows a dark line for your median glucose and shaded zones around it representing where your readings fell most of the time. When interpreting the AGP, follow a three-step order. First, look for dips below range, especially overnight or in the late afternoon. Second, look for persistent rises above range, particularly after meals. Third, assess variability by checking how wide the shaded bands are. Narrow bands with a smooth median line indicate stable, predictable glucose. Wide bands suggest something is inconsistent, whether that’s meal timing, activity, medication, or sleep.
Post-Meal Spikes and What They Mean
After eating, blood sugar naturally rises. What you’re watching for on a CGM is how high it goes, how fast, and how long it stays elevated. The general target is to be below 180 mg/dL two hours after the start of a meal. On a CGM graph, a healthy post-meal curve looks like a hill: a steady rise that peaks around 60 to 90 minutes, then comes back down to near your pre-meal level within two to three hours.
If your post-meal readings regularly spike above 200 mg/dL or take more than three hours to come back down, that pattern is worth noting. Compare different meals side by side in your CGM data. You’ll often find that specific foods or food combinations produce dramatically different curves. White rice might send you soaring while a similar portion of lentils barely moves the needle. This kind of personalized feedback is one of the most practical uses of CGM data, because it lets you make food choices based on your own body rather than generic nutrition advice.
Glucose Variability: The Overlooked Metric
Average glucose and time in range get most of the attention, but variability matters independently. Two people can have the same average glucose while one stays between 90 and 160 all day and the other bounces between 50 and 270. The second person’s risk profile is very different.
The standard measure for this is the coefficient of variation, or CV, which most CGM reports display. An international consensus panel identified a CV of 36% as the cutoff for high variability and increased risk of dangerous lows. For people taking insulin, a tighter target of 33% or below is often recommended. Research in well-controlled type 1 diabetes found that keeping CV at or below 31% was effective for maintaining time below 54 mg/dL at less than 1%, and a CV at or below 29.2% virtually eliminated serious lows in people whose glucose management indicator was already 6.5% or lower.
In practical terms, if your CV is above 36%, your glucose is swinging widely enough to increase your risk for both highs and lows. Focus on consistency: regular meal timing, predictable carb amounts, and steady activity patterns all help bring variability down.
Overnight Patterns Worth Recognizing
Two common overnight patterns show up on CGM data, and they require different responses. Both cause high fasting glucose in the morning, but the overnight traces look different.
The dawn phenomenon is a gradual rise in glucose that starts in the early morning hours, typically around 3 a.m. to 4 a.m., and continues until you wake up. On a CGM graph, you’ll see glucose that’s normal or slightly elevated at 2 a.m. to 3 a.m. and then climbs steadily. This rise is driven by hormones your body releases in the pre-dawn hours to prepare for waking.
The Somogyi effect (rebound hyperglycemia) looks different. Glucose drops low in the middle of the night, often around 2 a.m. to 3 a.m., and then rebounds sharply upward. On a CGM trace, you’ll see a clear dip followed by a steep climb. This is your body overcorrecting for the nighttime low. The distinction matters because the dawn phenomenon may call for a different medication or meal strategy, while the Somogyi effect means the nighttime low itself needs to be fixed first. A CGM makes telling them apart straightforward since you can see the entire overnight curve rather than relying on a single fasting reading.
GMI and How It Relates to A1c
Your CGM report includes a Glucose Management Indicator (GMI), which estimates what your A1c would be based purely on your sensor readings. The formula is straightforward: GMI equals 3.31 plus 0.02392 multiplied by your mean glucose in mg/dL. So an average glucose of 154 mg/dL gives a GMI of about 7.0%.
Here’s the important nuance: GMI and lab A1c don’t always match. Only about 20% of people have a GMI and A1c within 0.1% of each other. The gap can be clinically meaningful, especially in type 1 diabetes. A1c reflects the lifespan of your red blood cells and can be skewed by conditions like anemia, kidney disease, or genetic hemoglobin variants. GMI reflects only what the sensor measured. Neither number is “wrong.” They’re measuring different things. If your GMI is consistently lower than your lab A1c (or vice versa), it’s useful information, not an error to dismiss.
Understanding the Sensor’s Limitations
CGM sensors measure glucose in the fluid between your cells (interstitial fluid), not directly in your blood. This creates a lag: when blood sugar is changing rapidly, the sensor reading trails behind by up to 15 minutes, though it’s typically less. The lag is most noticeable during fast rises after meals and rapid drops during exercise. If your reading feels off during a moment of rapid change, that delay is the likely explanation.
CGM accuracy standards require that at least 87% of readings fall within 20% of a corresponding blood glucose value across the device’s measuring range. That’s strong overall accuracy, but it means individual readings can occasionally be off. A sensor reading of 100 mg/dL could reflect a true blood glucose anywhere from roughly 80 to 120 mg/dL. This is why trends and patterns over hours and days are more reliable than any single number. If a reading doesn’t match how you feel, particularly during a rapid change or when you suspect a low, a fingerstick meter provides a more immediate snapshot of what’s happening in your blood right now.
Putting It All Together
The most effective way to review CGM data is to resist fixating on individual readings and instead look at the bigger picture over 7 to 14 days. Check time below range first. Then check time above range. Then look at variability. Use the AGP to spot recurring patterns at specific times of day, whether that’s a consistent post-lunch spike, a late-afternoon dip, or an overnight rise. Pair those patterns with what you know about your meals, activity, sleep, and stress to figure out what’s driving them.
The trend arrows help you make real-time decisions, like whether to eat a snack before a walk or wait before correcting a high that’s already dropping. The summary metrics (TIR, CV, GMI) help you evaluate how things are going overall and whether adjustments to your routine are working. Together, they turn a continuous stream of glucose numbers into information you can actually use.