Why Does Fasting Blood Sugar Fluctuate?

All my clients track their fasting blood glucose because it’s one of the best ways to monitor progress in becoming more insulin-sensitive. It’s a simple, objective number that helps gauge whether their body is handling glucose more efficiently over time. 

But over the last two weeks, multiple guys have noticed something frustrating: some mornings, their fasting glucose is randomly higher than average. 

Naturally, they feel disappointed—thinking they’ve done something wrong or that their progress is stalling. But should they be?

The short answer: No. 

A single high fasting glucose reading doesn’t mean you’re off track. In fact, these fluctuations are completely normal and expected. 

Here’s why your fasting blood sugar might be higher on certain days—even when you’re doing everything right.

1. Dawn Phenomenon

Between 3-8 AM, your body releases glucose into the bloodstream to prepare you for waking up. This is a natural process controlled by hormones like cortisol and growth hormone. If your body is still insulin-resistant, it may struggle to regulate this glucose release, leading to a higher morning reading.

2. Rebound Hyperglycemia (Somogyi Effect)

If blood sugar drops too low overnight—possibly due to fasting, intense exercise, or even medications—the body compensates by releasing stress hormones like glucagon, epinephrine, and cortisol. This can cause a spike in blood sugar by morning.

3. Poor Sleep & Stress

A bad night’s sleep or high stress levels trigger cortisol release, which increases insulin resistance. Even if your diet and exercise are on point, poor sleep alone can cause higher fasting glucose the next morning.

4. Late-Night Eating

What you eat—and when you eat—matters. If you had a late-night meal, especially one high in carbohydrates or unhealthy fats, your body may still be processing it overnight, leading to a higher fasting glucose reading.

5. Liver Glucose Dumping (Gluconeogenesis)

The liver has a habit of releasing glucose into the bloodstream, especially if it senses you’ve been fasting for a long time or if it perceives a need for energy. This process can lead to higher fasting glucose levels, even if you haven’t eaten anything.

6. Mild Dehydration

Lack of water can concentrate blood sugar levels, making readings appear artificially high. Staying well-hydrated helps prevent this (IE Drink half your bodyweight in ounces daily).

7. Inflammation & Recovery from Exercise

If you had an intense workout the day before, your body may be dealing with muscle repair and mild inflammation, both of which can increase insulin resistance temporarily. This is a normal part of recovery.

8. Inconsistent Fasting Duration

If one night you fast for 8 hours and another night for 12, your body may react differently. The liver and insulin response can vary depending on how long you've gone without food.

What Should You Do?

Instead of stressing over a single higher-than-average reading, focus on trends over time. If your fasting glucose is generally improving over weeks and months, you’re heading in the right direction. 

Occasional spikes are part of the process.

Key Takeaways:

• A high fasting glucose reading doesn’t mean you’re failing

• Various factors—hormones, sleep, stress, late meals, hydration—can impact fasting glucose

• Look at long-term trends rather than single data points

• If readings remain consistently high, investigate deeper (e.g., sleep, stress, meal timing, hydration).

If you’ve seen an unexpected spike in your fasting glucose, don’t let it derail you. Stick with the plan, track your progress over time, and trust that your body is adapting.

The goal isn’t perfection—it’s steady, long-term improvement.