I notice my CGM shows high glucose first thing in the morning even though I haven't eaten. Why?

This is the dawn phenomenon: a normal physiological rise in blood glucose in the pre-awakening hours driven by cortisol and growth hormone secretion. Both hormones rise in the early morning as part of the circadian awakening response, and both counteract insulin, causing a mild glucose rise even in the absence of food.

In metabolically healthy men, the dawn phenomenon typically raises fasting CGM glucose by 10–20 mg/dL over sleeping levels. It is physiologically normal and does not represent metabolic dysfunction. Where it becomes clinically relevant is in men with pre-diabetes or insulin resistance: in these men, the dawn rise is exaggerated, often reaching 110–125 mg/dL by morning, because their insulin response is insufficient to counteract the cortisol effect efficiently. Chronic cortisol elevation from poor sleep, high stress, or subclinical sleep apnea amplifies the dawn rise further. (Van Cauter et al., Diabetes, 1989)

Cardiologist's calibrated position, Solid (1) for the dawn phenomenon as a normal physiology. The exaggerated version in insulin-resistant men is well-documented.

What to do: If your fasting CGM glucose is consistently above 95 mg/dL on waking, add a fasting insulin test. If your sleep is poor or fragmented, improving sleep quality often reduces morning glucose without any dietary change.

For the full picture, read The CGM on Your Arm Doesn't Know Your Arteries.