Understanding the Dawn Phenomenon: Why Blood Sugars Rise While Fasting

In this video, the speaker explains the dawn phenomenon and why blood sugars can sometimes go up while fasting. When you don’t eat, your body releases glucose from storage as a source of energy. This is controlled by counter regulatory hormones like noradrenaline, growth hormone, and cortisol. The dawn phenomenon occurs in everyone, causing a surge of these hormones in the morning to prepare the body for the day. In some people with type 2 diabetes, blood glucose levels can rise significantly in the morning even without eating. This is normal and indicates that the body is releasing stored glucose for energy. Fasting generally does not cause low blood sugars, but adjustments to diabetes medications may be necessary during fasting.

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How does this happen?

Key Insights:

  • When fasting, blood sugars normally go down, but in some cases, they may go up.
  • While fasting, the body releases glucose from storage sources to provide energy.
  • Counter regulatory hormones, such as noradrenaline, sympathetic nervous system, growth hormone, and cortisol, are responsible for releasing glucose during fasting.
  • The dawn phenomenon is a normal rise in blood glucose in the morning due to the release of counter regulatory hormones.
  • In people with type 2 diabetes, blood glucose may rise excessively in the morning, even without eating.
  • Fasting does not typically cause low blood sugars, as the body can maintain blood glucose levels through various mechanisms.
  • If taking medications for diabetes, adjusting medication dosage may be necessary during fasting to prevent hypoglycemia.
  • Studies have shown that even in prolonged fasting, individuals did not experience symptoms despite very low blood glucose levels due to the brain’s utilization of ketones as an energy source.
  • Overall, fasting can be beneficial in allowing the body to use stored glucose and improve blood glucose control.


Today we’re going to talk about the dawn phenomenon and what happens to your blood sugars while you’re fasting. Normally, when you’re fasting or not eating, you would expect that your blood sugars would go down. But sometimes they go up, and people are a little puzzled about it. I’m going to explain why that happens and how this is related to the dawn phenomenon.

So let’s start with the blood glucose. When you’re measuring the blood glucose, you expect that if you’re not eating, it would fall because when you eat, it tends to go up. So when you don’t eat, you would think that it goes down, and that’s usually what happens. But sometimes people are surprised to find that even when they’re not eating, it goes up.

The reason is the hormones. When you don’t eat, your body is going to release glucose for energy for your body. During eating, you’re going to store those calories and you’re going to store sugar, which is a source of energy. You store it as glycogen in the liver. You can also store it as body fat. When you don’t eat, your body then has the ability to take the body fat or take the glycogen in the liver and turn it back into glucose or sugar and pump it back out into the blood. And that’s the reason you don’t die in your sleep every single night because we have this storage form or reservoir of energy, and energy is often in the form of blood sugar or blood glucose.

Think of it like a refrigerator. When you go to the supermarket, you can get food and you can put it on your dining room table. But you can also put some of those groceries away in the refrigerator. But you can see what’s on your dining room table, but you can’t see what’s in the refrigerator. There might be lots of food in the refrigerator or there might be none at all. So if all the food is gone from the table and you don’t go to the supermarket, you might think, „Wow, there’s no food left.“ But if there’s still food there, where did it come from? Probably, you got it from the refrigerator. If there’s nothing in the fridge, then of course, there is no food. The same thing happens with your blood glucose. You have sugar in the blood, but there’s usually too much of it when you’re eating. So your body’s going to store some of it away, just like you store some of those groceries in the refrigerator. And when you don’t eat, your body is going to signal through its hormones to start releasing some of that glucose back out, just like you might take food out of the refrigerator.

Those hormones are known as counterregulatory hormones. So they’re called „counter“ because they run counter to insulin. When you eat, insulin goes up. Your body wants to store that energy. When insulin goes down, when you’re not eating, other hormones go up. So these hormones are noradrenaline, the sympathetic nervous system, growth hormone, and cortisol. So all of these hormones go up, and they have the effect of taking the glucose that’s in storage and pumping it back out into the blood.

During fasting, you also have a rise in counterregulatory hormones. So when you’re not eating, your blood glucose may go up because the body has now the signal to start taking out the storage glucose, and it’s going to go up. This is also seen in the daytime, first thing in the morning, which is why it’s called the dawn phenomenon. The dawn phenomenon happens in everybody. People, when they’re just about to wake up, somewhere around four o’clock, five o’clock in the morning, the body, because of its circadian rhythm, releases a surge of counterregulatory hormones. And what it does is it takes the sugar from your storage systems, pumps it out into the blood, and gets you ready for the day ahead. That’s why you don’t have to eat first thing in the morning if you don’t want to because your body’s already prepped you up and gave you energy to get going.

The whole dawn phenomenon is actually normal. You can measure this in normal people, but usually, the rise in blood glucose is very small. In some people with type 2 diabetes, what they find is that the blood glucose rises very high in the morning, even though they didn’t eat. And this is a variation of the response that you get to the rising glucose when fasting. So if you find that your blood sugar is rising when you’re fasting, there’s nothing wrong with you. It doesn’t mean you did anything wrong. It just means that your body is releasing a lot more glucose, probably because it had a lot of glucose stored away. Just like if you have a fridge that’s filled to overflowing, once you get that signal to take some food out, you’re going to take a lot out. So that’s what the body does too when it gets those counterregulatory hormones and gets the signal to release glucose. It releases a lot, and the blood glucose goes too high.

The other question about blood glucose sometimes I get asked is whether or not fasting causes low blood sugars. And the answer is no. Your body has the ability to keep your blood glucose in a normal level, no matter how much you fast. And that’s because it can turn the glycogen in the liver or the body fat back into sugar. In extreme cases, it can also take proteins in your body and turn that into glucose as well. That’s called gluconeogenesis. In a person who takes no medications for diabetes, they should never really develop low blood sugars in response to fasting. On the other hand, if you are taking medications for diabetes, remember that during fasting, you’re allowing the body to use up some of the body’s glucose. So as it uses the blood glucose, if you are also taking medications to lower that blood glucose, it can actually go too low. And that’s why it’s really important to talk to your doctor anytime you change your diet to make sure that you don’t need to make adjustments to your medications.

Some people think, „Well, that’s a reason why type 2 diabetics or people on medication shouldn’t fast.“ But that’s not exactly true. It just means you need to be careful when you’re fasting, and if your blood glucose tends to go too low, then you need to reduce your medications. You’re overmedicated. But remember, when you’re reducing your medications for blood glucose, that means your diabetes is actually getting better. So that’s a good thing, not a bad thing. It means you need to adjust that medication.

Many years ago, they did several studies looking at the question of whether fasting itself could cause low blood glucose. And it was a very interesting study because they took men and they fasted them for 30 to 60 days. And then, not only that, but they gave them a large bolus of intravenous glucose to drive that blood glucose down even further. In this study, they gave three people who had a long fast 20 units over 24 hours of insulin during the fasting period. You could see that their blood sugars were fairly normal, 5.5 to 4.5, pretty normal. But with that medication, they were able to drive that blood glucose down to about 0.9, which is a level where we get really nervous, and often people get seizures. But interestingly enough, the people who were fasted did not even feel a single thing. They were completely asymptomatic. They felt normal, even though their blood sugars were really low down, so low that we’d probably put them in the ICU. Why was that? It’s because they had a lot of ketones. The ketones are a source of energy for the brain, and they go up during fasting. When you’re fasted, your brain is mostly using ketones. So even if the blood glucose goes down very low, well, that was a small proportion of the energy that it needs, and most of its energy is coming from ketones anyway. So therefore, people felt very normal. So yet another reason why the body is able to tolerate those low blood glucoses even better while fasted.

In this other study from 1972, they did something very similar. They took nine people and put them on a 60-day fast. And then they gave them, once again, intravenous insulin to drive that blood glucose down just to see what would happen. And in fact, every single one of them didn’t notice anything at all. Those blood glucoses dropped down so low that it was almost unmeasurable in many cases, yet people felt completely normal.

So remember, in terms of the blood glucoses while you’re fasting, usually, it’s going to go down. Even if it goes down, it usually stays in the normal range. If you’re on medications, you need to make adjustments to those medications and speak to your doctor. And if it’s too low, then you may need to stop it completely. But in most cases, it’s not going to go so low that you’re actually going to feel it. Sometimes, however, the blood glucose goes up during fasting, and that’s not abnormal. And what you need to understand is that what it indicates is that your body probably has too much glucose overall. So the fasting is doing a good thing. What you’re doing is you’re allowing your body to release the glucose into the bloodstream so you can use it. It may temporarily go high, but that’s just glucose that’s coming out from your own body anyway. It’s just a chance for you to use it. So you actually are doing the right thing. You need to continue and keep going until you get that blood glucose back down.

Thanks for watching, everybody. I hope you learned something. If you did, please share it with somebody. They may learn something too. And if you could do me a favor and hit that like button so that other people can find that video, I’d really appreciate it. Thanks for watching, and I’ll see you next week.