What Is a CGM and Why Does It Matter?
A Continuous Glucose Monitor (CGM) is a small sensor worn on your body that measures your glucose levels every few minutes, 24 hours a day. Unlike traditional finger-prick tests that give you a single snapshot, a CGM provides a full picture of how your glucose moves throughout the day.
For people living with Type 1 Diabetes, this continuous stream of data can be very helpful. It reveals patterns that finger pricks simply cannot catch, including overnight lows, post-meal spikes, and the subtle effects of stress, exercise, and sleep on your glucose levels.
Reading the Trend Arrows
Your CGM displays trend arrows alongside your current glucose number. These arrows are just as important as the number itself:
Steady (flat arrow): Your glucose is stable with no significant rise or fall. This is the goal for most of the day.
Rising slowly (single up arrow): Glucose is climbing at about 1-2 mmol/L per 15 minutes. This might be normal after a meal, but it is worth monitoring.
Rising quickly (double up arrow): Glucose is spiking fast, at more than 2 mmol/L per 15 minutes. Consider whether you need a correction dose or if a recent meal is still being absorbed.
Falling slowly (single down arrow): A gradual decline. If you are already in range, keep an eye on it but do not panic.
Falling quickly (double down arrow): Glucose is dropping fast. Have fast-acting carbs ready and check again in 15 minutes. If you are about to exercise or drive, treat first.
Understanding Time in Range (TIR)
Time in Range is one of the most useful metrics your CGM provides. It tells you the percentage of time your glucose stays within your target range, typically 3.9 to 10.0 mmol/L (70 to 180 mg/dL).
The goal for most people with T1D: - Time in Range: above 70% - Time below range: less than 4% - Time above range: less than 25%
Do not chase perfection. A TIR of 70% means roughly 17 hours per day in range, which is excellent management. Focus on gradual improvement rather than overnight transformation. Even a 5% improvement in TIR has been shown to meaningfully reduce the risk of long-term complications.
Spotting Patterns Over Time
The real power of CGM data comes from looking at trends over days and weeks, not just the current number.
Morning highs (Dawn Phenomenon): If your glucose consistently rises between 4 and 8 AM without eating, your liver may be releasing stored glucose. Talk to your endocrinologist about basal rate adjustments.
Post-meal spikes: If you spike after lunch every day, it might be time to pre-bolus earlier or adjust your insulin-to-carb ratio for that meal.
Exercise dips: If you go low 2 hours after every workout, try reducing your bolus before exercise or having a small snack beforehand.
Overnight patterns: Look at your overnight trace. Is it flat, rising, or dipping? This tells you whether your basal insulin dose is dialed in.
Practical Tips for Getting Started
If you are new to CGM, here are some tips to avoid overwhelm:
1. Do not react to every number. Look at the trend, not just the point. A glucose of 11 that is falling is very different from an 11 that is rising.
2. Set reasonable alerts. Start with wider alert thresholds and tighten them as you get comfortable. Too many alarms leads to alarm fatigue.
3. Review weekly, not hourly. Sit down once a week to look at your patterns in the CGM app summary reports. This is where the real insights live.
4. Share with your care team. Most CGM apps let you share data with your endocrinologist or diabetes educator. This makes appointments far more productive.
5. Be patient with yourself. CGM data can feel overwhelming at first. It takes most people 2 to 4 weeks to get comfortable interpreting the information. Give yourself grace during the learning curve.