Dawn Phenomenon Explained: Why Fasting Blood Sugar Is High in the Morning

Diabetes Science · 5 · February 19, 2026

You went to bed at 110 mg/dL. You slept eight hours. You did not sleepwalk to the refrigerator. You wake up, test: 162. Where did those 52 extra milligrams per deciliter come from?

Your liver made them. Between 4 AM and 8 AM, your body releases cortisol, growth hormone, glucagon, and epinephrine. These hormones tell your liver to dump stored glucose into your blood — a wake-up call from evolution designed to give you energy for the morning hunt. In people without diabetes, the pancreas counters with insulin. In people with diabetes, the insulin response falls short. Morning glucose rises.

This is the dawn phenomenon. It affects 50–75% of people with Type 2. It is not caused by what you ate last night. It is caused by your hormonal alarm clock.

Dawn Phenomenon vs Somogyi Effect

There is a second reason for high morning glucose that looks identical: the Somogyi effect. If your blood sugar drops too low overnight — usually from too much evening insulin — your liver panics and dumps glucose to prevent hypoglycemia. Result: high morning glucose. But the cause is the opposite.

Test at 2–3 AM to tell them apart. Normal or high at 3 AM? Dawn phenomenon. Below 70 at 3 AM? Somogyi effect. The distinction matters because the treatments are opposite — more medication for dawn phenomenon, less for Somogyi.

A CGM makes this trivially easy. Connect your Libre or Dexcom to Journey for Health's CGM Monitor and look at your overnight trend line. The answer is staring at you in the graph.

What Helps

Eat dinner earlier — finishing by 7 PM reduces morning glucose by 15–25 mg/dL in most patients. Walk for 15 minutes after dinner. Consider extended-release metformin at bedtime instead of with dinner. A tablespoon of apple cider vinegar in water before bed reduced fasting glucose by 4–6% in a small Diabetes Care study. Not dramatic, but real.