Fasting Glucose Explained: How Blood Sugar Can Change Your Body & Brain

Understand how maintaining optimal fasting glucose can have a positive impact on your body and brain.

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By Staff Writer
Caitlin Snethlage
Edited by Caitlin Snethlage

Published March 6, 2024.

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The misconception about fasting glucose is that many believe it's a measure used to prevent diabetes only, rather than something measured in the process of improving your mind and body.

In this blog, we explain why fasting glucose is important, why you should get tested, and provide an argument for why you should pay attention to this vital part of your metabolism every day.

Key Takeaway

  • Higher fasting blood glucose levels may be associated with poor mood and depression.
  • Fasting blood glucose that is too low may impair your arousal.
  • Habitual sleep debt and disturbed sleep may reduce insulin sensitivity and increase fasting blood glucose levels.
  • Increasing sleep time may improve glucose levels, insulin levels, and insulin sensitivity.
  • A higher fasting blood glucose level in the normal range may be a risk factor for decreased cognition and brain health.
  • Lower fasting blood glucose can impair memory, attention, and processing of audio and visual information; and it may lead to aggression.
  • Glucose management should be a lifetime commitment to maintain a healthy mood, sleep, and cognition.  

Glucose explained 

Blood glucose accumulates in our blood primarily from the carbohydrates we eat. The hormone, insulin, allows glucose to enter cells, so insulin is critical to helping the body use glucose properly.

a poster describing the benefits of glucose

Properly functioning insulin sensitivity means smaller amounts of insulin are needed to reduce blood glucose levels.

Insulin resistance develops when cells don’t respond to insulin. This results in a buildup of blood glucose, or hyperglycemia, which raises your fasting blood glucose.

However, when implementing dietary strategies to reduce fasting blood glucose, it’s important to avoid reaching low blood glucose, or hypoglycemia (this occurs when blood sugar levels drop below our optimal range). Signs such as sweating, increased heartbeat, and jitteriness will worsen if you don’t take action. Feelings of confusion, loss of consciousness, and coma can ensue if glucose levels are too low.

What is fasting blood glucose?

A blood glucose test (which InsideTracker offers) is used to determine your fasting blood glucose level. Fasting blood glucose is the level of glucose (sugar) in the blood after an overnight fast.

The test requires you to fast for at least 12 hours, and it can be done any time of the day. The easiest way to get tested is to do it right after you wake up.

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Importance of monitoring fasting blood glucose levels

When it comes to health, being “normal” just might not be good enough. Your best bet is to strive for optimal. Health practitioners emphasize that we must maintain our fasting blood glucose (sugar) level in the "normal range."

We’re repeatedly reminded of the health consequences of glucose levels above the normal range (risk for type 2 diabetes, metabolic syndrome, etc.). However, truly healthy blood glucose levels may not necessarily be in the entire scope of the "normal range" for every person. We should focus not just on reaching the normal, but reaching the optimal fasting blood glucose level—because that does more for our health than originally thought. 

Fasting glucose is linked with mood, sleep, and cognition. Optimizing your blood glucose (better than having a "normal" level) may be the best bet for maintaining sharpness throughout the day and feeling better overall.

» Read more about hemoglobin A1c (HbA1c) test

Fasting blood glucose ranges and your score

The fasting blood glucose test is usually done to screen for prediabetes and diabetes. But this test offered by InsideTracker is important for everyone because it suggests how well your body is regulating glucose.

  • Normal blood sugar range: 65-99 mg/dL
  • Prediabetes, or impaired fasting glucose: 100 to 125 mg/dL
  • Diabetes: 126 mg/dL or higher on two consecutive tests
a screen shot of the insidetracker app with a bar graph for blood glucose

Is there anything wrong with these ranges? Yes, because the ranges can be arbitrary. Blood sugar at high levels starts impacting not just your body tissues, but also other factors like mood, sleep, and cognition.

Should you care about improving your blood sugar when you’re in the normal range? 

Yes! It’s not just about the risk for certain metabolic diseases (i.e. diabetes). And there’s not a big difference between 97 mg/dL in the normal range versus 100 mg/dL in the prediabetes range; hence, arbitrary. 

What is the ideal fasting glucose?

The normal range for blood sugar is a wide window—so what’s optimal? Several studies suggest the ideal fasting glucose is less than 90 mg/dL, and most likely somewhere around 85 mg/dL.[1, 2] Major cardiovascular and cancer risks may occur at levels above 85 mg/dL. [1, 3, 4]

At fasting glucose levels above 85 mg/dL, glucose concentrations come into contact with body tissue, which can lead to glycation or chemical reactions with body protein and fat. These glycated proteins and fats cause inflammation and oxidative stress, which negatively impacts aging. This is why InsideTracker offers the InnerAge test, so you can monitor key biomarkers, like glucose, that contribute to aging.

Optimizing fasting glucose may also help glucose control throughout the day. Let’s say a person has a reading of 95 mg/dL: this may not truly reflect the person’s all-day glucose status, which could actually be higher. Following eating, your blood glucose level should not considerably increase above the fasting level. So the lower, or more optimal score your fasting glucose is, the more manageable blood glucose will be after eating.

Association between fasting glucose and mood, sleep, and cognition

Avoiding the glucose rollercoaster to improve your mood

Can a lower fasting glucose level reduce your risk of depression or simply put you in a better mood? The co-occurrence of mood disorders and diabetes (which is high fasting blood glucose) is well established. [5] There is a direct relationship between diabetes and depression. [6, 7] The severity of a person’s diabetes appears to be associated with the severity of their depression.

study used 57 overweight/obese men with and without type 2 diabetes to evaluate associations between depressed mood and glycemic control. When analyzing both groups together, it was found that depressed mood was correlated with fasting glucose. 

Higher fasting blood glucose levels were associated with increased symptoms of depression. It was suggested that improving glycemic control might not only yield important clinical benefits but also psychological benefits. It is questionable as to whether or not this is the case for women and other demographic groups. 

  • Low blood sugar can cause neurological symptoms such as mood swings.
  • Signs of hypoglycemia will begin to appear when blood sugar drops too low.
  • Even mild mood changes, like getting easily annoyed, may be a sign. 
  • Blood glucose levels that vary even within the normal range, can interact with emotional processing. [7]

study using 20 healthy female participants explored the effects of blood glucose levels after overnight fasting on mood regulation. All participants had a fasting glucose of less than 80 mg/dL before each underwent a functional MRI (magnetic resonance imaging) scan, which observes different parts of the brain or other body organs that are active.

The study found that prolonged fasting appeared to decrease the subjective feeling of arousal (acts as a cue for mood), [8]  which can influence your attention span. Epinephrine levels were lower in the fasting state compared to the elevated blood glucose state. Lower epinephrine accompanying low fasting blood glucose was suggested as the cause for a lower arousal state.

Both extremes of the spectrum of blood glucose have negative impacts on mood. The benefits of good glucose control will result if you avoid the highs and lows. 

Sleep your way to better glucose levels

Sleep plays a pivotal role in glucose metabolism.

A person’s habitual decreased sleep time and increased disturbed sleep are strongly suggested to be factors impacting the development of type 2 diabetes. [9] Studies have found that a decrease in sleep (or "sleep debt") yields a decrease in insulin sensitivity when compared to completely rested. [10, 11]

Prolonged sleep debt can impact insulin levels. A study found from a single fasting blood test, that five nights of sleep restriction to four hours per night, resulted in a major increase in insulin levels, yet not a significant change in glucose concentration. This generated an increase in the insulin-to-glucose ratio, which is suggested to be a biomarker for insulin resistance.

Another study analyzing 11 young healthy men, tracked both 6 days of sleep restriction (4 hours in bed), followed by full sleep recovery (12 hrs in bed per night for 6 nights). During sleep debt, the participants had higher levels of blood glucose following breakfast.

Sleep debt and disturbed sleep can negatively impact glucose metabolism. However, this can be reversed by sleep extension or simply adding more sleep time.

An intervention study used 16 healthy, sleep-restricted, non-obese people to investigate if sleep extension would have a positive impact on glucose metabolism. The intervention consisted of two weeks of their usual time in bed followed by 6 weeks of increasing time in bed by one hour per day.

Glucose and insulin were tested following habitual time in bed and after the sleep extension. Glucose levels, insulin levels, and insulin sensitivity all improved.

Improving your insulin sensitivity can help control your fasting blood glucose and subsequently take control of your blood sugar.

a mockup of the InsideTracker app with Sleep duration screen

Increase your cognitive performance with optimized glucose levels  

A higher, normal blood glucose level, may be a risk factor for decreased cognition and brain health. 

study compared 210 cognitively healthy people without diabetes, glucose intolerance, or metabolic syndrome to determine if fasting blood glucose levels in the normal range were associated with cognition.

The study found that higher glucose levels in the normal range were associated with less grey matter (which processes information in the brain) and white matter (which carries nerve pulses like the brain’s subway) in the brain, and the consequential lower grey and white matter were associated with poorer cognitive performance. 

This was specifically found in the frontal part of the brain, which is associated with aging and memory impairments. This is important because other research linking diabetic glucose levels with cognitive performance means that this association is also seen in what’s considered “normal” fasting glucose levels.

The results had an interesting gender component:

  • In women, only smaller grey matter regional volumes were significantly associated with the speed of processing information.
  • Yet, in men, smaller grey matter regional volumes were significantly associated with worse information processing speed, memory, and executive function, or mental skills that help you get tasks done.

Decreased language function was associated with smaller white matter regional volumes.

Even though these were people between 68 and 73 years old, the results emphasize the point that managing glucose is a lifetime commitment, especially in protecting ourselves from cognitive consequences as we age.

Yet, low blood glucose can also have a broad impact on cognition. [12]

In healthy adults, cognitive complications can arise at levels between 45-55 mg/dL. [13] Tasks that are more complex are more strongly impacted than smaller tasks. Memory, attention, and processing of visual and audio information are also impaired. These consequences can still continue even when glucose levels are restored.

Low fasting blood glucose is associated with aggression in men. [14} Yet in women, it can lead to greater anger. [15]

To keep your brain performance high, maintain optimal fasting glucose.

Know and improve your fasting glucose by testing 

It’s best to test and not to guess. Knowing your blood sugar level will help you optimize it. InsideTracker provides the tools for you to reach your optimal fasting glucose level so you can preserve your mood, sleep, and cognition.

Even better, repeated testing with InsideTracker can reveal your blood sugar trends. Optimize your fasting blood sugar so that you can take control and set yourself up for a high-performance future.




3. The Expert Committee on the Diagnosis and Classification of Diabetes Mellitus. Report of the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus. Diabetes Care. 2002;25(1):s5-s20.





8. Clark, M.S., Milberg, S., Ross, J. Arousal cues arousal-related material in memory: Implications for understanding effects of mood and memory. J Verbal Learning Verbal Behav. 1983;22(6),633-649.