Why Is My Blood Sugar High After Fasting for 16 Hours?

Your liver is still actively pumping glucose into your bloodstream, and your body’s early-morning hormonal surge is amplifying that output. This process, known as the dawn phenomenon, is the most common explanation for elevated fasting blood sugar after 16 hours without food. But it is not the only one. Underlying insulin resistance, poor sleep, dehydration, and even your genetics can all push morning readings higher than you expect, sometimes in combination.

The Dawn Phenomenon

Between roughly 3 a.m. and 8 a.m., your body prepares for the day by releasing a burst of hormones, mainly growth hormone and cortisol. These hormones signal your liver to increase glucose output so your brain and muscles have fuel before you eat. In people without diabetes, a matching rise in insulin quietly offsets the extra glucose. If your insulin response is even slightly blunted, glucose accumulates in the bloodstream with nowhere to go.

Research on the dawn phenomenon in people with type 1 diabetes has shown that nocturnal growth hormone spikes alone are enough to drive roughly a 30 percent increase in the rate at which the liver releases glucose, along with about a 25 percent drop in how efficiently muscles and other tissues take up that glucose. When those growth hormone spikes were experimentally suppressed, the dawn rise in liver glucose output disappeared entirely, and when the spikes were reintroduced, the rise returned right on schedule.¹

The dawn phenomenon is not limited to people who already have diabetes. Anyone with prediabetes or mild insulin resistance can experience a meaningful bump in fasting glucose during these early-morning hours. If your 16-hour fast ends at, say, 7 or 8 a.m., you are checking your blood sugar right in the middle of this hormonal peak. That timing alone can explain a reading that feels confusingly high.

Your Liver Never Actually Stops Making Glucose

A common misconception about fasting is that your body simply burns through its stored glucose and then waits patiently until you eat again. In reality, your liver is manufacturing new glucose the entire time you fast, a process called gluconeogenesis. It converts amino acids, lactate, and glycerol into glucose and steadily releases it into your blood. This is a survival mechanism: your brain depends on a minimum level of circulating glucose at all times, and your liver ensures the supply never runs out.

The hormone glucagon is a major driver of this process. As insulin levels fall during fasting, glucagon rises and tells the liver to ramp up glucose production. Under conditions of metabolic stress, glucagon can push liver glucose output even higher to meet the body’s perceived demand.²

In someone with good insulin sensitivity, the system stays balanced. The liver makes just enough glucose, and the small amount of insulin circulating in the background is sufficient to keep blood sugar from rising. But in people with hepatic insulin resistance, the liver does not respond properly to insulin’s “slow down” signal. It keeps producing glucose at a higher rate than necessary, even when blood sugar is already elevated.³ Research comparing people with impaired fasting glucose to those with normal fasting glucose found that the impaired group had higher rates of gluconeogenesis, even though their bodies were producing more insulin to try to compensate.⁴

The uncomfortable truth for people practicing intermittent fasting is that a longer fasting window can sometimes mean the liver has been running this gluconeogenesis program for longer, with fewer brakes on the process. If you already have some degree of insulin resistance, 16 hours without food gives that unchecked glucose output a long runway.

Poor Sleep and Stress Drive It Higher

If you slept poorly the night before checking your fasting blood sugar, that is a likely contributor. Sleep deprivation triggers the sympathetic nervous system, the same fight-or-flight system that responds to acute stress. One consequence of that activation is increased nighttime cortisol secretion. Cortisol directly promotes insulin resistance and impairs the pancreas’s ability to regulate glucose, leading to higher blood sugar by morning.⁵

The effect does not require dramatic sleep loss. Even a night of lighter, more fragmented sleep compared to your usual pattern can raise cortisol enough to move the needle on morning glucose. Chronic short sleep, the kind many people live with and consider normal, has been linked to higher rates of impaired fasting glucose in large population studies. If you are fasting for 16 hours but sleeping five or six of them, you are working against yourself.

Psychological stress operates through the same cortisol pathway. A stressful evening, an argument before bed, or waking up anxious in the middle of the night can all produce a cortisol response that mirrors sleep deprivation. Your liver treats that cortisol signal as a reason to make more glucose, compounding whatever the dawn phenomenon was already doing.

Dehydration Is an Overlooked Factor

During a 16-hour fast, many people also go long stretches without drinking enough water, especially overnight. Research in people with type 2 diabetes has found that dehydration alone produces higher blood glucose levels compared to being well-hydrated. In one study, fasting glucose was measurably higher in a mildly dehydrated state, and the difference persisted and widened after a glucose challenge. The dehydrated group also had elevated cortisol that stayed higher throughout the test, suggesting part of the glucose increase was driven by a stress-hormone response to low fluid intake.⁶

Drinking water, herbal tea, or other non-caloric fluids throughout your fasting window does not break the fast. If you tend to stop drinking fluids after dinner and then do not drink again until you check your glucose the next morning, that simple habit change could lower your readings. The mechanism is partly about blood concentration (less water in the blood means the same amount of glucose registers as a higher concentration) and partly about the cortisol pathway mentioned above.

Does Your Eating Window Timing Matter?

People who practice 16:8 intermittent fasting typically eat either in an early window (roughly 8 a.m. to 4 p.m.) or a later window (roughly noon to 8 p.m.). You might expect the timing to affect next-morning glucose, but the evidence on this is nuanced and, frankly, a bit deflating.

A trial comparing early versus late time-restricted eating found no significant differences in fasting blood glucose between the two approaches.⁷ Similarly, a 12-week study of time-restricted eating in overweight adults without diabetes found no change in HbA1c, the measure that reflects average blood sugar over about three months.⁸

There is a bright spot for early eating, though. A study in adults with prediabetes found that eating earlier in the day reduced the amount of time spent in an elevated glucose range during daytime hours by about 4 percent compared to eating on an unrestricted schedule. However, the early eating window did not improve overnight glucose levels at all.⁹ So if your concern is specifically morning fasting glucose, shifting your eating window earlier is unlikely to fix the problem on its own. The dawn phenomenon and overnight liver glucose production are running on hormonal clocks that eating timing does not easily override.

Fasting Exercise Can Temporarily Raise Blood Sugar

If you work out in the morning before eating, especially at high intensity, your blood sugar can actually climb during and after the session. A study in people with type 1 diabetes found that high-intensity interval exercise performed in a fasted state caused blood glucose to rise from about 7.6 to 9.9 mmol/L during and after exercise, while the same workout done after eating produced a gradual decline.¹⁰

The mechanism is straightforward: intense exercise triggers a release of adrenaline and glucagon that tells the liver to flood the bloodstream with glucose. When you have recently eaten, insulin from that meal counterbalances the surge. When you are fasted, there is less circulating insulin to offset it, so glucose goes up temporarily. If you are checking your blood sugar right after a fasted morning workout and wondering why it is high, that is likely why. The rise is transient, and over the course of a day, regular exercise still improves insulin sensitivity. But the timing of your reading matters.

The Somogyi Effect Has Largely Been Debunked

For decades, a common explanation for high morning blood sugar was the Somogyi effect, the idea that an episode of low blood sugar overnight triggers a rebound surge of counter-regulatory hormones that overcorrects, pushing glucose too high by morning. This theory has been taught in medical schools and repeated in diabetes education materials for years. The problem is that modern data does not support it.

A study using continuous glucose monitoring in people with type 2 diabetes found no evidence for the Somogyi effect. The data showed that low overnight glucose readings were not followed by rebounds to high morning glucose. Instead, overnight lows simply predicted low morning glucose, and high morning glucose tracked with high readings throughout the night.¹¹ If your morning fasting glucose is high, the most likely explanation is not a rebound from an overnight low. It is the dawn phenomenon, ongoing liver glucose production, or both.

How to Figure Out What Is Going On

If your fasting blood sugar is consistently elevated after 16-hour fasts, the first useful step is figuring out whether the dawn phenomenon is the primary driver. The classic approach is to check your blood glucose between 3 a.m. and 5 a.m. for several nights. If your glucose is normal at 3 a.m. and clearly higher by the time you wake up, the dawn phenomenon is likely responsible. If your glucose is already elevated at 3 a.m., the issue is more about persistent overnight glucose production and insulin resistance.¹²

A continuous glucose monitor makes this easier, since it tracks your glucose automatically throughout the night and shows the full overnight trend without multiple finger sticks. If you do not have access to one, even two or three middle-of-the-night checks can give you a useful picture.

Practical steps that address the most common causes include:

Staying hydrated: Drink water or non-caloric fluids throughout your fasting window, including before bed and first thing in the morning.
Prioritizing sleep: Seven or more hours of uninterrupted sleep reduces the cortisol surge that amplifies morning glucose output.
Timing exercise carefully: If you exercise fasted, check your glucose before and after to understand how your body responds. A moderate walk tends to lower glucose, while intense intervals may raise it temporarily.
Talking to your doctor: Consistently elevated fasting glucose, meaning readings above about 100 mg/dL, warrants a conversation about insulin resistance, prediabetes, or medication timing if you are already being treated.

If you are not diabetic and are doing intermittent fasting for general health, a modestly elevated fasting glucose reading does not necessarily mean something is wrong. Your body may be in a state of mild, transient glucose elevation because your liver is doing exactly what it evolved to do: keeping your blood sugar from crashing during a long fast. The reading is a snapshot of an active regulatory process, not a sign that fasting has broken something.

Melatonin, Genetics, and Individual Susceptibility

There is a genetic dimension to morning fasting glucose that most people are unaware of. Researchers have identified variation in the gene for a melatonin receptor (MTNR1B) as a risk factor for both impaired fasting glucose and type 2 diabetes.¹³ Melatonin, the hormone that makes you sleepy at night, appears to interact with insulin signaling in the pancreas. When melatonin levels are high and a glucose load arrives at the same time, people with certain MTNR1B variants may have a harder time producing enough insulin to handle it.

This matters for intermittent fasters because melatonin levels are still elevated in the early morning hours. If you are genetically susceptible, the overlap between lingering melatonin and the dawn phenomenon’s glucose surge may produce a larger spike than it would in someone with different gene variants. You cannot change your genetics, but understanding this helps explain why two people following the same fasting protocol can get very different morning readings.

What Your Gut Bacteria May Have to Do With It

An emerging area of research connects the gut microbiome to how your body handles glucose during and after fasting. Certain gut bacteria produce short-chain fatty acids, particularly propionate and butyrate, as they ferment dietary fiber. These compounds do more than just feed the cells lining your intestine. They stimulate a process called intestinal gluconeogenesis, where the gut itself produces small amounts of glucose that are sensed by receptors in the nearby portal vein. This gut-to-brain glucose signal helps regulate appetite, supports early-phase insulin release, and influences how the liver manages its own glucose production.¹⁴

The practical implication is still being worked out, but the direction is clear: the composition of your gut bacteria and the amount of fiber in your pre-fast meals may influence how well your body regulates glucose overnight. A diet low in fiber means fewer short-chain fatty acids, which may mean weaker intestinal signals telling your liver to dial back glucose production. This is an area where the research is young enough that specific dietary recommendations would be premature, but it suggests another reason why what you eat before your fast matters, not just when you eat.

Does the Menstrual Cycle Affect Fasting Glucose?

Women who track their blood sugar sometimes notice that their readings seem to fluctuate across the menstrual cycle, leading to a reasonable suspicion that hormonal shifts between the follicular and luteal phases affect insulin sensitivity. The research here is more reassuring than most people expect. Studies measuring insulin sensitivity across different menstrual phases have found no significant differences, either between phases or between repeat measurements taken within the same phase.¹⁵ The variability in a given person’s glucose readings over time was similar regardless of where they were in their cycle.

This does not mean that individual women cannot experience perceptible differences. Progesterone does have mild effects on glucose metabolism, and some women with diabetes report needing small insulin adjustments in the luteal phase. But for the question of why fasting blood sugar is high on a particular morning, menstrual cycle phase is unlikely to be the main explanation. The factors discussed earlier, especially the dawn phenomenon, sleep quality, and hydration, carry far more weight.