The Dawn Phenomenon: High Morning Sugar

The dawn phenomenon is a rise in blood sugar in the early morning, before you have eaten anything — and it is the reason many people find their fasting reading is the highest of the day.

Between roughly 3 am and 8 am, your body releases a surge of hormones including cortisol and growth hormone, which tell your liver to push out glucose ready for the day. In someone without diabetes the pancreas covers it. If your insulin is insufficient or is not working well, the glucose stays in the blood and you wake to a high number.

It is not unusual. Studies suggest it affects more than half of people with Type 1 or Type 2 diabetes.

Telling it apart from a rebound

There is a second, less common cause of morning highs: a low overnight that triggers the same counter-regulatory hormones, sending you high by morning. This is the Somogyi effect, and it matters because the two need opposite responses — more insulin overnight for one, less for the other.

The only reliable way to tell them apart is to know what happened at 2 or 3 am. A CGM shows this directly. Without one, checking at that hour for a few nights is the manual version.

See also understanding overnight numbers.