What is the dawn effect, and how does it affect blood sugar?

Early morning blood sugar spikes are closely linked with metabolic illnesses like diabetes. Here’s what you should know about the dawn effect.

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If you’re monitoring your blood sugar, you may notice that it rises during the early morning hours before you’ve done anything that might normally cause a rise, like eating. In people with diabetes, where this has been mostly studied since being initially discovered in 1979, it’s known as the dawn effect, or the dawn phenomenon. But it is now being studied in broader populations as well. Anecdotally, many Levels members without diagnosed metabolic impairment see this effect when wearing CGMs. In this article, we’ll explain what we know about the physiology of this phenomenon and what you can do about it.

What is the Dawn Effect?

Researchers generally define the dawn effect as an elevation in blood glucose (hyperglycemia) beginning anywhere between 3-4 am and 8 am or as an increased need during those hours for external insulin among people who use it to control their blood glucose levels (such as people with diabetes). Unlike typical glucose spikes, the dawn effect is not a direct result of eating, stress, or exercise: It’s a spontaneous increase in your fasting blood sugar.

After you eat breakfast, however, the effect may become even more pronounced, causing your blood glucose levels to reach higher levels than you might expect from the meal alone. This is sometimes called the Extended Dawn Phenomenon, and it’s a more reliable indicator of the dawn effect than pre-breakfast hyperglycemia. This was demonstrated by a 2013 study in which researchers used continuous glucose monitors (CGMs) to track the blood sugar of 248 people with Type 2 Diabetes over two consecutive days. They then separated participants into two groups: those who exhibited the dawn effect and those who did not. The two groups saw the biggest difference in the post-breakfast period.

As we will explore below, the dawn effect is most well-studied in people with diabetes (both Type 1 and Type 2), but it is also seen in people without diabetes. There isn’t a universally accepted set of glucose or insulin values that define the dawn effect, but one recent review defines it as:

  1. A blood glucose climb of 10-30 mg/dL or more from its lowest point during the night to just before breakfast, or
  2. A blood glucose climb of 30-60 mg/dL or more from its lowest point during the night to just after breakfast, or
  3. A more than 20% increase in the amount of exogenous (medical) insulin needed to control blood sugar

Other studies have specified timeframes for this change (e.g., a 20 mg/dL rise between 5 am and 7 am) or included proxy metrics for blood glucose (such as C-peptide).

How Common is the Dawn Effect and Who Experiences It?

Historically, it’s been difficult for researchers to pin down the prevalence of the dawn effect, with early estimates ranging from 6% to 90% of people with diabetes. This is largely due to the challenge of identifying blood glucose highs and lows via individual “snapshot” tests, which only provide one data point and are inconvenient to administer during sleep. However, the increased use of continuous glucose monitors has helped kickstart a new era of research into the dawn effect, and estimates have gradually converged over the past decade. One recent small study suggested that the phenomenon affects about 55% of people with Type 1 Diabetes and 52% with Type 2 Diabetes.

Dawn effect research in people without diagnosed diabetes is even more nascent, but some studies are emerging. A recent study in China examining blood glucose data from 781 participants found that the dawn effect occurs in people without diabetes and in people who do not yet have diabetes but have impaired glucose regulation:

Prevalence of dawn effect by metabolic health status:

  • No Diabetes: 8.9%
  • Impaired Glucose Regulation: 30.1%
  • Newly Diagnosed Type 2 Diabetes: 52.4%

Additional research shows that the dawn effect may be both more common and more extreme among people with impaired glycemic control. A 2016 study of people with Type 2 diabetes in China found that the dawn effect was more common among people with higher BMI, which is associated with poor metabolic health. The researchers divided 98 people into three groups: people with normal weight (BMI 18.5-23.9), people who were overweight (BMI 24-27.9), and people with obesity (BMI 28 or higher). During a 72-hour window, 33.3% of people with normal weight experienced the dawn effect. Among people who were overweight, that number was 78.8%. It rose to 88.6% among people with obesity. The study also found that the dawn effect was more common in people with greater insulin resistance, larger waist circumference, and higher levels of triglycerides.

Similarly, a 2021 study of 50 people with Type 2 diabetes found that the dawn effect was linked to glucose “excursions”—the spikes in blood sugar that typically follow a meal. The more intense a person’s blood sugar excursions in general, the more likely they were to experience the dawn effect.

What Causes the Dawn Effect?

Growth Hormone

The dawn effect is caused by several interrelated factors, the most important of which is growth hormone (GH) secretion. Your body releases growth hormone in pulses during the night, especially as you enter deep sleep. GH helps build and maintain muscles and bone tissue. But GH also causes your liver to release glucose into your bloodstream (a process called endogenous glucose production, or EGP). Simultaneously, GH makes your body less sensitive to insulin—a key metabolic hormone that helps shuttle glucose into your cells, where it is used for energy. In other words, your natural glucose production rises while your ability to use that glucose decreases.

Growth hormone reduces your insulin sensitivity via several pathways. First, it triggers lipolysis—the release of fatty acids from the adipose, or fat, tissue. These fatty acids circulate in the bloodstream and make your muscles more insulin resistant, decreasing the glucose being taken up by your cells. GH also directly reduces the liver’s response to insulin, causing it to release even more blood sugar from stored carbs and further reducing glucose uptake in the rest of the body. 

In people without diabetes, the body may typically counter this GH effect with a surge of insulin, which keeps blood sugar excursions low. But in people with diabetes, the pancreatic cells which secrete insulin (called beta cells) cannot produce enough insulin to keep blood sugar in check. The body enters hyperglycemia, remaining there until insulin can overcome the effects of GH and bring blood sugar back down. This “tug of war” between GH and insulin seems to be the driving force behind the dawn effect.

Evidence that GH is one cause of the dawn effect began accumulating with a 1985 study of six people with insulin-dependent diabetes (also known as Type 1 diabetes, when people require insulin injections because they cannot produce sufficient insulin on their own). When researchers administered glucagon and a drug called somatostatin, which blocks growth hormone, they found that the dawn effect disappeared. When they replaced the missing growth hormone with hourly GH injections (still using somatostatin). A later study of eight people with Type 1 diabetes reached the same conclusion: Blocking the secretion of GH eliminated the dawn effect. Yet another 2003 study of young adults with Type 1 diabetes was in alignment, finding that a GH-blocking medication reduced the need for overnight insulin—a hallmark of the dawn effect.

Circadian Rhythm Disruptions

Another possible contributor to the dawn effect is an altered circadian rhythm, which seems to appear in people with diabetes. Studies have shown that the liver’s blood sugar production occurs in a distinct daily pattern among people with diabetes that causes a 30-50% EGP elevation in the morning. This pattern is not typically seen in people without diabetes, and we don’t fully understand its origin. It may be due to changes in the brain’s suprachiasmatic nucleus, which regulates EGP and the sleep-wake cycle. Disruptions to this internal “clock” are linked to the dawn effect in people with diabetes.

Cortisol

Finally, the dawn effect may also be caused partly by a nighttime surge of another hormone: cortisol. Sometimes called a “stress hormone,” cortisol blood levels begin rising between 2 am and 3 am and peak at around 8:30 am. Like growth hormone, cortisol counters the action of insulin. It reduces whole-body insulin sensitivity, raising your blood sugar. This makes energy readily available to your muscles, helping you wake up and get active in the morning. But if you cannot produce enough insulin to counter the cortisol’s effects, it could theoretically lead to the dawn effect.

The science is mixed on whether cortisol plays a meaningful role in the dawn effect. One 1990 study of seven people with “non-insulin-dependent diabetes” (equivalent to Type 2 diabetes) found that reducing cortisol significantly lowered morning blood glucose levels, and some contemporary researchers consider the link plausible. Other studies found that blocking cortisol does not prevent the dawn effect, leading some diabetes researchers to rule out the hormone as a culprit. The issue has not been settled conclusively.

What Can You Do About Dawn Effect?

Although high glycemic variability is best avoided for optimal metabolic health, whether or not the dawn effect presents unique risk is unclear, though it may be more significant in people with diabetes. One 2013 study of 248 people with non-insulin-treated diabetes found that the dawn effect worsened participants’ overall glycemic control by about 0.4% on an HbA1c test and about 12 mg/dL in 24-hour mean glucose. This impact could not be mitigated with any of the available diabetes treatments. In a review of 30 years of dawn effect research, the authors write, “Because it is an early metabolic abnormality leading over time to the vicious circle of ‘hyperglycemia begets hyperglycemia’…the dawn phenomenon in T2D should be treated early and appropriately.”

For people without diabetes, there is not yet sufficient research to know whether the dawn effect represents a health risk, nor how you might counter it. But in general, research suggests that minimizing glycemic variability (big spikes and dips in your daily blood sugar levels) can promote better metabolic health. Here are some strategies that can help:

  • Prioritize your sleep. Extensive research links poor sleep quality to insulin resistance, diabetes, and other metabolic health issues. Practice good sleep hygiene by keeping your bedroom fully dark and quiet, winding down at the same time every night, and only using your bed for sleep (no TV or phone).
  • Exercise regularly. Exercise is a pillar of metabolic health. In one review, people who did moderate aerobic exercise for 30 minutes three times per week saw a significant boost in insulin sensitivity after just eight weeks. Another recent study on people with diabetes found that a moderate-intensity aerobic workout before breakfast partially countered the dawn effect
  • Avoid late meals. A recent study found that people who ate dinner at 6 pm had significantly better glycemic control during the following 24 hours than people who dined at 9 pm. 
  • Reduce breakfast carbs. Foods that are high in carbohydrates can spike your blood sugar almost immediately. That includes refined grains (breakfast cereals, toast, bagels), fruit juice, and sweetened beverages like tea and coffee. Mixing protein and fat into your carbs can help slow digestion and reduce your post-breakfast blood glucose spike.
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