A continuous glucose monitor measures interstitial glucose every 1-15 minutes via a subcutaneous filament sensor (typically worn on the arm or abdomen for 10-14 days).
Mechanism — Sensor enzyme reaction translates glucose concentration to electrical signal. Reads interstitial glucose, which lags blood glucose by ~10-15 minutes during rapid changes. Reveals meal responses, sleep glucose patterns, stress excursions.
Use case — 2-4 week trials reveal personal glucose response patterns. Diabetics use continuously; metabolic-health optimizers use periodically. Brands: Dexcom, FreeStyle Libre, Stelo, Nutrisense.
Caveats — Don't chase flat lines. Postmeal excursions to 140-160 mg/dL are normal; over 180 is the concern. Single-CGM data is noisy — focus on patterns over weeks. Numbers vary by sensor.