If you wear a continuous glucose monitor, the number that matters most usually isn’t today’s single high or low reading — it’s time in range (TIR): the percentage of the day your glucose sits inside a target band. As a dietitian, TIR is one of the first things I look at with a client, because it tells a food story that a single fasting number never can. Here’s how to actually read it, and how it connects to what’s on your plate.
What Time in Range Actually Measures
Time in range is exactly what it sounds like: the share of your CGM readings, usually over a rolling 14-day window, that fall inside a defined glucose band. It’s reported alongside two companion metrics — time below range (TBR) and time above range (TAR) — and together the three should add up to 100% of your day. Where a lab A1c gives you a single blended average, TIR shows the shape of your day: whether you’re steady after breakfast but spiking after dinner, or drifting low mid-afternoon before your usual snack.
This is why TIR has become the metric clinicians and consensus guidelines lean on alongside A1c: two people can share the same A1c while one spends the day in smooth, narrow swings and the other bounces between highs and lows that average out to the same number. TIR surfaces that difference.
The Target Numbers Worth Memorizing
International consensus guidelines, echoed by the American Diabetes Association, set specific targets for most adults with diabetes. They’re worth writing down, because they’re what your CGM app’s report is measuring you against:
| Metric | Range | Target |
|---|---|---|
| Time in Range (TIR) | 70–180 mg/dL | At least 70% of the day (roughly 17 hours) |
| Time Below Range — Level 1 | 54–69 mg/dL | Less than 4% of the day |
| Time Below Range — Level 2 | Below 54 mg/dL | Less than 1% of the day |
| Time Above Range — Level 1 | 181–250 mg/dL | Less than 25% of the day |
| Time Above Range — Level 2 | Above 250 mg/dL | Less than 5% of the day |
These are general starting points, not a one-size-fits-all pass/fail grade — a clinician will typically loosen the low-end targets for someone at higher hypoglycemia risk, or tighten the high-end target during pregnancy. But as a baseline, if your TBR is creeping toward that 4% ceiling, that’s a food-timing and portion conversation worth having before it’s a medication conversation.
What a Healthy Post-Meal Trend Line Actually Looks Like
Time in range is a 24-hour average, but the food-specific information lives in the shape of each post-meal curve. A few things worth knowing about how a normal response typically behaves:
- Peak timing: glucose usually peaks 30 to 60 minutes after a meal begins, not immediately.
- Peak height: in someone without diabetes, post-meal glucose typically stays under 140 mg/dL; the general treatment target for someone managing diabetes is to stay under 180 mg/dL after eating.
- Return to baseline: a smooth response comes back down to pre-meal levels within roughly 2 to 3 hours.
- Shape: a “fast spike, fast recovery” — a clean rise and a clean fall — is generally the sign of a well-tolerated meal.
The pattern worth watching for is the opposite shape: a rise that doesn’t come back down within that window, or a second smaller rise appearing after the first one has already started to fall. Meals that are heavier in fat or protein alongside the carbohydrate are the most common cause of this — they slow gastric emptying, which can turn what should be a single clean peak into a delayed, prolonged “tail,” or even a second bump hours later as digestion catches up. That tail matters for TIR because it’s time spent above range that a same-size, lower-fat version of the same meal might not have produced.
Reading the Trend, Not Just the Number
The most useful habit I can hand a client isn’t a single target to hit — it’s learning to read their own trend line against their own meals. A few patterns worth checking your own data for:
- Same meal, different response. Sleep, stress, activity earlier in the day, and even the order you eat food in a meal can all shift how the same plate reads on a CGM. One flat response and one spike from the “same” dinner isn’t a contradiction — it’s useful data about what else was going on that day.
- Where the time above range clusters. If TAR is concentrated in a two-hour window after one specific meal, that’s a far more actionable finding than a vague sense that “the number was high yesterday.”
- Overnight and fasting drift. A pattern of low-normal readings that dip toward the bottom of range overnight is worth flagging distinctly from daytime post-meal patterns — the causes, and the fixes, are usually different.
None of this replaces a conversation with the clinician managing your care, particularly around adjusting medication in response to what the trend shows. But reading time in range as a pattern — rather than treating each reading as its own verdict — is what turns a CGM from a source of daily anxiety into a genuinely useful piece of nutrition feedback.
The Takeaway
Time in range condenses an entire day of glucose data into one number, but the number is only the headline. The real information — the part that actually changes what you eat and when — is in the shape of the curve after each meal: how fast it rises, how high it goes, and how long it takes to come back down. Learn to read that shape alongside the target ranges above, and time in range stops being a score to chase and starts being a genuinely useful conversation with your own food.