The morning high glucose mystery: why is it high when I wake up?

What is the dawn phenomenon?

The dawn phenomenon is a natural rise in blood glucose that happens in the early morning, usually between about 3 am and 8 am. This is the way your body prepares you for the day. This happens for everyone, but in someone without diabetes, the pancreas simply releases a bit more insulin to match the glucose rise and nothing changes. In people with diabetes, the insulin-matching response doesn't happen as effectively, so glucose levels rise.

If this is something you have noticed, it is not related to anything you have done ‘wrong’. It's a predictable, well-studied pattern, and it's something you can identify and act on.

What causes it?

Your body runs on an internal clock called the circadian rhythm, and part of that clock's job is to prepare you to wake up and function. In the last few hours before dawn, roughly 3 am to 8 am, your body releases a cocktail of hormones as part of this "wake-up" programming: These include

• Cortisol
• Growth hormone
• Glucagon
• Adrenaline

These hormones all have one thing in common: they tell the liver to release more glucose into the bloodstream and make the body's cells temporarily less responsive to insulin. In people without diabetes, the insulin output quietly increases to soak up that extra glucose. If your body can't produce or deliver enough insulin at that time, glucose builds up and you see it on your morning reading.

It's not just about the fasting reading

The dawn phenomenon can affect glucose levels before breakfast and after breakfast.

This is important because you may wake up with a slightly high glucose level and then see it climb even further after breakfast.

Regularly waking up with high glucose levels can add up - nudging your overall average, or HbA1c reading, higher over time.

Is it really the dawn phenomenon?

A high morning glucose level doesn't automatically mean it’s caused by the dawn phenomenon.

There are several possible reasons for waking up with a high glucose level.

How can you check?

The easiest way is to look at what happens to your glucose during the night, rather than just comparing bedtime with waking.

A Continuous Glucose Monitor (CGM) can be useful in this situation because it allows you to see the whole overnight pattern.

For example:

Pattern 1: Possible dawn phenomenon

Glucose levels in range at bedtime → stable overnight → gradual rise in the early morning → high on waking

Pattern 2: Glucose was already high overnight

Glucose levels high at bedtime → remains high overnight → high on waking

These are different problems and may need different approaches.

What can you do about it?

There isn't one solution that works for everyone. The right approach depends on what's causing the rise and what treatment you're already on.

1. Look at the whole overnight pattern

Before changing anything, find out what your glucose is doing overnight.

2. Review your evening meal

A large meal, particularly one high in carbohydrate or fat, or eating close to bedtime can cause glucose to remain elevated for several hours.

3. Consider physical activity

Regular physical activity can make you more sensitive to insulin and help with your glucose management. For some people, an evening walk after dinner can help reduce post-meal glucose levels.

4. Look after your sleep

Poor sleep can affect hormones involved in glucose regulation and may increase insulin resistance.

If you snore heavily, wake frequently, gasp during sleep or feel excessively tired during the day, it's worth discussing this with your healthcare professional as a sleep study may be recommended.

5. Review your diabetes medication

Sometimes the answer is not simply "eat differently."

Your medication may need to be adjusted to better match your glucose pattern. Talk to your doctor or diabetes care team before changing any medication or insulin dose.

What else can affect your morning glucose?

Plenty of things beyond dawn phenomenon can shift your morning number:

• Food - large, late, high-carb or high-fat evening meals (fat delays the glucose rise), and bedtime snacks
• Medication - missed or mistimed doses, insufficient overnight insulin, or changing insulin needs
• Sleep - too little, disrupted sleep, shift work, or sleep apnoea
• Stress - physical or emotional stress raises glucose via stress hormones
• Illness - infections and other illness increase insulin requirements
• Alcohol - affects glucose unpredictably, depending on the amount, timing and other factors
• Exercise - improves insulin sensitivity, but the effect varies with type, timing and your medications
• Routine changes - travel, shift work, or an altered sleep/wake schedule can all shift your patterns
• “Leaky liver” (hepatic insulin resistance) - a fatty liver becomes less responsive to insulin's signal to stop producing glucose, so it continues to release glucose into the blood overnight even when it shouldn't, raising your fasting glucose independent of any hormone surge
• Somogyi effect (rebound hyperglycaemia) - if glucose drops too low overnight (hypoglycaemia), the body releases hormones to push it back up, leaving you high by morning

The most important thing to remember

If your glucose is higher when you wake up, don't automatically assume you've done something wrong.

First, look at the pattern.

Ask: What was my glucose doing overnight?

Once you understand the pattern, you and your diabetes healthcare team can decide whether changes to food, activity, sleep, medication or insulin are appropriate.

The goal isn't to chase one high number. It is to understand what your body is doing overnight and choose a treatment that works with that pattern.