A CGM (Continuous Glucose Monitor) is a small skin sensor that measures glucose every few minutes for 10 to 14 days. The sensor sits on the back of your arm and sends data wirelessly to your phone, so you can see how your blood sugar reacts in real life.

Do I need a prescription for a CGM?

You usually need a prescription for medical-grade CGMs like Dexcom or Libre, although some over-the-counter sensors (Stelo, Lingo) are now available. Even with an over-the-counter sensor, we recommend physician guidance to interpret the data.

How long do you wear a CGM?

You wear each sensor for 10 to 14 days, depending on the brand. For a baseline metabolic audit at Fishtown Medicine, we usually use one or two sensors back-to-back, for a total of 14 to 28 days of data.

A CGM does not hurt for most people. The sensor uses a tiny filament under the skin that you cannot feel during normal activity. The application takes a few seconds and feels like a quick pinch.

Can I shower with a CGM?

Yes, you can shower with a CGM. Modern sensors are waterproof and stay in place during showering, swimming, and exercise. Some patients use a sticker overlay for extra security during heavy training.

How accurate are CGMs?

CGMs are accurate for the trends and patterns we use clinically. They are slightly behind a finger-stick blood test in real time because they measure interstitial fluid, not blood. The trend is what matters most for metabolic decisions.

What is a normal blood sugar range?

A normal fasting blood sugar is roughly 70 to 99 mg/dL, and most healthy people stay below 140 mg/dL after meals. On a CGM, we look for steady, gentle curves and limited time above 140 mg/dL or below 70 mg/dL.

Will a CGM help me lose weight?

A CGM can help with weight management by showing which foods cause your biggest spikes and crashes, and by linking those crashes to hunger and snacking. Pair the data with a structured nutrition plan, and many patients eat less by accident because their hunger drops.

What is glycemic variability and why does it matter?

Glycemic variability is the size of the swings between your highest and lowest blood sugar during the day. High variability is linked to inflammation, fatigue, and worse cardiovascular outcomes, even when the average (A1C) looks normal.

How does a CGM detect early insulin resistance?

A CGM can detect early insulin resistance by showing prolonged spikes after meals and slow returns to baseline. Insulin resistance is when cells stop responding well to insulin, the hormone that moves sugar into cells. The CGM reveals this years before A1C rises into the prediabetic range.

Why does my blood sugar spike when I am stressed?

Your blood sugar spikes when you are stressed because cortisol and adrenaline tell the liver to release stored glucose. This is helpful for short-term threats, but during chronic work stress it produces unnecessary glucose loads, which can show up clearly on a CGM.

How does sleep affect blood sugar?

Sleep affects blood sugar because even one bad night can lower insulin sensitivity by 20% to 30% the next day. CGMs often show flatter, healthier curves after a good night and more chaotic spikes after a poor one, which makes sleep changes feel concrete.

What is the dawn phenomenon?

The dawn phenomenon is a small, natural rise in blood sugar between roughly 4 a.m. and 8 a.m., driven by morning cortisol and growth hormone. A modest rise is normal. Large rises can suggest early insulin resistance and deserve further evaluation.

How does exercise change CGM patterns?

Exercise changes CGM patterns by lowering glucose during and after activity. A 10-minute walk after a meal can drop a typical spike by 30 to 50 mg/dL. Strength training and zone 2 cardio (steady moderate-effort cardio) improve overall insulin sensitivity over weeks.

Can a CGM diagnose diabetes?

A CGM can suggest a diabetes pattern, but it does not replace formal diagnostic testing. Diabetes is diagnosed with fasting glucose, A1C, or an oral glucose tolerance test. A CGM is most useful for finding earlier metabolic changes before those tests are abnormal.

What is the difference between fasting glucose and CGM readings?

Fasting glucose is one number from one moment, usually drawn first thing in the morning. CGM readings are continuous, so you see the full daily pattern. The two are complementary, not interchangeable.

How does alcohol affect CGM data?

Alcohol affects CGM data by lowering blood sugar overnight, sometimes into the 50s and 60s mg/dL. This drop can disturb sleep, drive cravings, and worsen the next day's glucose response. Many patients see this clearly only after one weekend on the sensor.

Are CGMs worth the cost for healthy adults?

CGMs can be worth the cost for healthy adults who want a focused 14 to 28 day audit, particularly if they have brain fog, weight stalls, or sleep issues. Long-term use is usually not necessary; the goal is to learn your patterns, then apply the lessons.

How does Fishtown Medicine choose between Dexcom, Libre, and Stelo?

Fishtown Medicine chooses between Dexcom, Libre, and Stelo based on your goals, budget, and whether you need real-time alerts. Dexcom and Libre have the strongest clinical track record. Stelo is over-the-counter and useful for basic education in healthy adults.

What happens after my 14-day CGM audit?

After your 14-day CGM audit, we review the data together and translate the patterns into a personalized plan. That plan usually includes meal adjustments, exercise timing, sleep tweaks, and (when appropriate) targeted lab work or medication.

The CGM Guide: High-Resolution Metabolism

TL;DR30-second take

A CGM (Continuous Glucose Monitor) is a small skin sensor that tracks your blood sugar every few minutes for 10 to 14 days. Non-diabetics use it to see how meals, sleep, and stress shape energy and long-term metabolic health, often years before standard labs would show a problem.

TL;DR: A CGM (Continuous Glucose Monitor) is a small wearable sensor that tracks your blood sugar every few minutes, 24 hours a day. At Fishtown Medicine, we use CGMs preventively to find the subtle metabolic changes that standard labs miss. You do not need diabetes to benefit from knowing how your body responds to food, stress, and sleep.

For driven adults in Philly, a CGM often answers a simple question: why am I tired at 3 p.m. even though I ate "healthy" food? The data usually tells a clear story, and the fix is much more specific than another generic diet plan.

What is a CGM?
Why Non-Diabetics Use Them
What It Shows That Labs Don't
Guidelines from the Clinic
Common Questions
Deep Questions

What is a CGM?

A CGM (Continuous Glucose Monitor) is a discreet sensor that sits on the back of your arm. It measures your interstitial glucose, which is the sugar in the fluid between your cells, and sends that data directly to your phone. It is waterproof, nearly painless, and lasts for 10 to 14 days.

Why do non-diabetics use a CGM?

Non-diabetics use a CGM to fine-tune energy, sleep, and long-term metabolic health, not just to manage disease. We use CGMs for patients who want to improve their longevity and performance.

Energy Optimization: Find the crashes that cause afternoon brain fog.
Sleep Architecture: See if overnight glucose dips are waking you at 3 a.m.
Glycemic Variability: Track how chaotic or stable your blood sugar is across the day.
PCOS & Insulin Resistance: Catch early changes in metabolism years before they show up on an A1C test (the 90-day average blood sugar).

What does a CGM show that standard labs do not?

A CGM shows the moment-to-moment behavior of your blood sugar, while standard labs only show an average. Your A1C may look "perfect" at 5.2%. A CGM may still reveal that you spike to 180 mg/dL after a "healthy" oatmeal breakfast, then crash to 65 mg/dL two hours later. These spikes and crashes drive inflammation and hunger, and they are invisible to standard one-time blood tests.

Guidelines from the Clinic

"A CGM is a flight recorder for your metabolism. It removes the guesswork from nutrition. Instead of following a generic diet, you can see exactly how your biochemistry reacts to a bowl of rice versus a piece of sourdough. Data builds momentum. When you see a spike line up with your fatigue, you do not need willpower to make a change. You just need the data."

How We Use the Data

Macro-Balancing: We help you adjust your protein and fiber intake to blunt the glucose response of your favorite meals. A macronutrient is a major energy source: protein, fat, or carbohydrate.
Exercise Timing: We show how a 10-minute walk after a meal can sharply lower a glucose spike.
Stress Awareness: We identify how a stressful meeting at work drives your glucose up even when you have not eaten.

Actionable Steps for Metabolic Clarity

Master your daily energy.

Perform a 14-Day Audit: Wear a sensor for two weeks to set your metabolic baseline.
Test Your Staples: Eat your usual breakfast and watch the app. If you spike above 140 mg/dL, we adjust the macros.
Check Your 3 a.m. Wakes: If your glucose drops sharply before you wake up, we may need to adjust your evening meal or Sleep Strategy.
Walk After Meals: Test the same meal with and without a 10-minute walk after eating. The CGM will show you the size of the difference.

Key Takeaways

CGMs provide real-time data, not just snapshots.
They are used for energy, sleep, and prevention, not only diabetes.
They help identify glycemic variability (the spikiness of your blood sugar).
We use the data to build your precision nutrition plan.

Scientific References

Battelino T, et al. "Clinical Targets for Continuous Glucose Monitoring Data Interpretation." Diabetes Care. 2019.
Hall H, et al. "Glucotypes reveal new patterns of glucose dysregulation." PLOS Biology. 2018.
Reynolds AC, et al. "Impact of five nights of sleep restriction on glucose metabolism, leptin and testosterone in young adult men." PLOS One. 2012.
Buffey AJ, et al. "The Acute Effects of Interrupting Prolonged Sitting Time in Adults with Standing and Light-Intensity Walking on Biomarkers of Cardiometabolic Health." Sports Medicine. 2022.

Dr. Ash is a board-certified internal medicine physician specializing in preventive medicine and healthspan optimization at Fishtown Medicine in Philadelphia.

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