A 38-year-old woman with type 1 diabetes is found to have a fasting plasma glucose of 180 mg/dL despite being on insulin. She reports that her blood glucose is normal at bedtime but elevated each morning. Continuous glucose monitoring shows a nadir at 2–3 AM followed by a rise. Which mechanism best explains this pattern?

• A Somogyi effect: nocturnal hypoglycemia triggering counter-regulatory hormone surge
• B Dawn phenomenon: growth hormone and cortisol rise in early morning causing insulin resistance ✓
• C Waning insulin effect: inadequate insulin dose by morning
• D Reactive hyperglycemia from excessive evening carbohydrate intake

Explanation

Continuous glucose monitoring showing a nadir at 2–3 AM (not frank hypoglycemia) followed by a progressive rise indicates the dawn phenomenon, driven by growth hormone and cortisol surges in the early morning hours that increase hepatic glucose output and reduce peripheral insulin sensitivity. The Somogyi effect (option A) requires actual hypoglycemia at the nadir; its existence as a clinically relevant phenomenon is debated. Waning insulin (C) would show a gradual overnight rise without a nadiring pattern.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.