Dawn Phenomenon: Why Blood Sugar Rises in the Morning
The dawn phenomenon is a rise in blood sugar in the early morning, caused by a normal surge of hormones, that happens without a low beforehand. It is not caused by something you ate, and it is common enough that clinicians know it well.
What happens
In the hours before waking, the body releases more cortisol, growth hormone, adrenaline and glucagon. Together they tell the liver to release glucose so you have energy to get up. They also make the body’s cells a little less responsive to insulin. In someone without diabetes, the pancreas simply makes more insulin and the glucose level barely moves. In someone with diabetes, the body may not make enough extra insulin, or its medicine may be wearing off, so glucose rises. Reviews describe this as happening in the early hours, roughly 3 to 8 a.m.
Dawn phenomenon or a rebound?
A high morning reading has more than one explanation, and the right response differs, so it helps to know which it is.
| Dawn phenomenon | Rebound after a night low (Somogyi effect) | |
|---|---|---|
| What comes first | Nothing: glucose drifts up in the early morning | A low during the night, then a hormone-driven rise |
| How to tell | Overnight readings stay steady, then rise | Overnight readings dip below 70 mg/dL (3.9 mmol/L) first |
| Who decides what to do | Your care team, with your overnight data. Please do not change doses yourself. | |
The rebound explanation is the more debated one, and some studies found that morning highs after a night low are less common than once thought. The point for you is practical: do not guess. A continuous glucose monitor shows the whole night. Without one, your care team may suggest a check around 2 or 3 a.m. for a few nights.
What to bring to your appointment
- A week of morning readings with the time and what you ate and did the evening before. Use the blood sugar log.
- Any overnight readings or CGM graphs.
- Notes on symptoms: morning headaches, night sweats or nightmares can hint at an overnight low.
- Your questions: Is this the dawn phenomenon? Should medicine timing or type change? What is my morning target?
What research says about it
The dawn phenomenon was described in the early 1980s in studies of people with type 1 and type 2 diabetes. Later work linked the rise to nighttime surges in growth hormone in type 1, and to extra glucose production by the liver with inadequate insulin action in type 2. Reviews report it is common and linked to a higher A1C, but there is no agreed definition when using CGM data, so rates vary. It does not mean your diabetes is “failing”. It means a normal daily rhythm is meeting a body that cannot fully offset it.
Looking at the wider day helps, too. The ADA lists time in range alongside A1C. Try the GMI calculator to see how your readings spread across the day, and read about fasting blood sugar ranges.
Frequently asked questions
What is the dawn phenomenon?
A rise in blood sugar in the early morning hours, before breakfast, caused by a natural surge in hormones such as cortisol, growth hormone and adrenaline that signals the liver to release glucose. In people without diabetes, insulin offsets the rise. In people with diabetes it may not.
How is the dawn phenomenon different from the Somogyi effect?
The dawn phenomenon is a rise that is not preceded by a low. In the Somogyi effect, a low during the night is followed by a rebound high. A CGM or a middle-of-the-night check can show which happened.
What time does the dawn phenomenon happen?
The rise is usually described as occurring in the early hours of the morning, roughly 3 to 8 a.m. The exact timing varies from person to person.
Can I stop it myself?
Please do not change medicine or doses on your own. Your care team can look at your overnight pattern and decide whether anything should change, which may include medicine type or timing, and is individual.
Does everyone with diabetes have it?
No. Studies have found it in many people with type 1 and type 2 diabetes, but how often depends on how it is defined and measured, and there is no agreed CGM definition.
Information only, not medical advice. Your care team may set different targets for you, and only a lab test ordered by a clinician can diagnose diabetes or prediabetes. See our medical disclaimer.
Sources
- Dawn Phenomenon. StatPearls, NCBI Bookshelf
- Monnier L et al. Thirty Years of Research on the Dawn Phenomenon. Diabetes Care 2013
- ADA Standards of Care in Diabetes—2026, Section 6: Glycemic Goals, Hypoglycemia, and Hyperglycemic Crises
Last reviewed: by the Diabetes Blogs editorial team. Information only, not medical advice.
