Why is my blood sugar high in the morning? 4 common causes

You went to bed with a good number, didn’t eat anything overnight, and your blood sugar was still high when you woke up. It’s one of the most common and most frustrating patterns in diabetes. There’s usually a reason, and there are four main suspects.

1. The dawn phenomenon

In the early morning, your body releases hormones such as cortisol and growth hormone that tell your liver to make more glucose. It’s your body getting you ready for the day.

Without diabetes, the pancreas simply releases more insulin to cover this. With diabetes, that extra insulin doesn’t come, so blood sugar rises. The American Diabetes Association says this typically happens between about 3 and 8 a.m., and that roughly half of people with type 1 or type 2 diabetes experience it.

The dawn phenomenon is the most common cause of high morning blood sugar.

2. A rebound after a low at night (the Somogyi effect)

The idea here is that blood sugar drops too low overnight, and the body responds with a surge of hormones that pushes it back up, too far. You wake up high without knowing you were ever low.

Is the Somogyi effect real?

It’s debated. Cleveland Clinic describes the Somogyi effect as a theory and says the dawn phenomenon is a much more common cause of morning highs. Research using continuous glucose monitors (CGMs) is mixed:

• A 2022 study in people with type 2 diabetes found no evidence of the Somogyi effect.
• A 2025 study of 755 people with type 1 diabetes found that highs following a low at night were frequent, especially in younger people.

So it’s less common than was once thought, but for people who take insulin it’s worth ruling out, because a morning high after a low at night is a very different situation from the dawn phenomenon.

3. Your insulin wearing off overnight

Sometimes the answer is simpler: the background insulin isn’t lasting until morning. The ADA notes this can happen if basal insulin isn’t enough overnight, or if long-acting insulin is taken early in the day and doesn’t last until the next morning.

4. A late dinner or bedtime snack

What you eat in the evening can still be affecting you hours later (see carb counting for beginners). The ADA points out that a large dinner or a bedtime snack can keep blood glucose raised all night.

Dawn phenomenon vs. Somogyi effect: how to tell which one it is

The causes can look identical at 7 a.m. The difference is what happens during the night. The ADA suggests checking your blood sugar:

• at bedtime,
• in the middle of the night,
• and first thing when you wake up.

Here’s roughly how the patterns differ:

What you see overnight	Likely cause
Steady through the night, then rising in the early morning	Dawn phenomenon
Dropping low in the night, then high by morning	Rebound after a low (Somogyi effect)
Slowly drifting up all night	Insulin wearing off
High from bedtime and staying high	Evening meal or snack

If you use a CGM, you may already have this data. Cleveland Clinic calls CGM the most effective way to spot the dawn phenomenon. New to CGMs? Here’s how a CGM works. It’s also helpful to keep a note of when you took your insulin and when you ate, so you can line them up against the overnight curve.

High in the morning but normal the rest of the day?

That’s a very common pattern, and it fits the dawn phenomenon well: the hormone rise the ADA describes happens in the early morning, roughly between 3 and 8 a.m. But a low overnight or insulin wearing off can look the same by breakfast. The overnight readings are what tell them apart.

Talk it through with your care team

Each cause calls for a different response, and some point in opposite directions. A high after a low at night, for example, is not a reason to take more insulin. That’s why it’s worth bringing a few nights of readings to your diabetes team rather than adjusting things yourself. As Cleveland Clinic puts it, if your blood sugar is consistently high in the morning, talk to your healthcare provider.

The short version

• Morning highs usually come from one of four causes: the dawn phenomenon, a rebound after a low at night (the Somogyi effect), insulin wearing off, or evening food.
• The dawn phenomenon is the most common, affecting about half of people with diabetes.
• You can’t tell the causes apart from the morning number alone. Look at what happens overnight.
• Take a few nights of readings to your care team before changing anything.