What CGM Data Can Tell You (And When a Wearable Is Enough)

Continuous interstitial glucose (every 1-5 min), glucose variability, time in range, post-meal spikes, fasting baseline, overnight patterns, exercise response. Nothing else.

Heart rate, HRV, sleep stages and duration, body temperature, blood oxygen, respiratory rate, activity and steps, readiness and recovery scores. No glucose.

Insulin, cortisol, testosterone, inflammatory markers, micronutrients, hydration status (accurately), lactate, or any other blood biomarker not derivable from glucose or PPG signals.

Your overnight fasting glucose reflects baseline insulin sensitivity and glycogen regulation. Standard lab reference is below 100 mg/dL, but functional medicine targets are tighter: 72-85 mg/dL is optimal, 85-100 mg/dL may reflect early insulin resistance even within reference range. A CGM captures this with precision a single fasting draw cannot: some people have stable fasting glucose while others have wild overnight variability, and those patterns are invisible without continuous monitoring.

How high your glucose rises after eating, and how quickly it returns to baseline. Most metabolic health guidelines consider post-meal glucose above 140 mg/dL as elevated, and above 180 mg/dL as a red flag. A healthy post-meal response peaks within 30-60 minutes and returns to baseline within 2 hours. Prolonged elevation (staying above 120 mg/dL for 3+ hours after eating) suggests impaired glucose clearance.

The percentage of time your glucose stays within a target band. Most CGM apps use 70-180 mg/dL as the standard range (the clinical target for T1 and T2 diabetes management), but metabolic optimization protocols use a tighter band of 70-140 mg/dL. Aiming for 90%+ time in a tight range is a reasonable target for non-diabetic metabolic health optimization.

Coefficient of variation measures how much your glucose fluctuates relative to your mean. High variability (above 36% CV) is associated with worse outcomes independent of average glucose level. A low average glucose with high variability (frequent spikes and crashes) is a worse profile than a slightly higher average glucose that is stable throughout the day. Variability reduction, not just average lowering, is a meaningful CGM target.

If your fasting glucose is above 90 mg/dL, your triglyceride-to-HDL ratio is above 2.0, or you have family history of T2 diabetes, a CGM provides early warning data years before standard labs would flag anything. The post-meal response in particular reveals glucose clearance impairment that a single fasting draw misses entirely.

Athletes who want to optimize fueling for training and competition benefit from seeing exactly how carbohydrate timing, meal composition, and pre-workout nutrition affect their glucose response. Understanding your personal glucose patterns around workouts enables more precise fueling decisions than any generic protocol.

Individual glucose responses to specific foods vary enormously between people with identical metabolic health. A food that causes a large spike in one person may be flat in another. CGM data lets you identify your specific high-spike foods, which often reveals patterns that are impossible to infer from general glycemic index tables. Two weeks of CGM data can reshape your entire understanding of how you respond to food.

If you have prediabetes or strong family history and are actively working on metabolic health, CGM provides real-time feedback on whether your interventions (diet changes, exercise, meal timing) are producing the desired glucose responses. It closes the feedback loop that periodic A1C tests cannot, which are only updated every 3 months and reflect average glucose, not pattern quality.

A chronically declining HRV baseline over 6-12 months without corresponding changes in training load, sleep, or other known stressors is a meaningful metabolic health signal. Insulin resistance impairs autonomic function through oxidative stress and advanced glycation end-products damaging vagal nerve function. But this effect takes years to manifest as a detectable HRV change.

Elevated resting heart rate (above 75 bpm at rest) that persists across weeks and is not explained by training load changes, illness, or dehydration suggests chronic sympathetic activation. Insulin resistance activates the sympathetic nervous system through free fatty acid elevation and impaired baroreceptor sensitivity. A creeping resting heart rate with no obvious cause is worth investigating metabolically.

Worsening sleep fragmentation and reduced slow-wave sleep percentage visible in wearable sleep staging can reflect nocturnal glucose variability from impaired glucose regulation. This signal is less specific (many causes) but more acute: dietary changes (large carbohydrate loads in the evening, alcohol) can produce detectable sleep fragmentation within a single night.