<p>The <a href="http://neuro.hms.harvard.edu/harvard-mahoney-neuroscience-institute/brain-newsletter/and-brain-series/sugar-and-brain" rel="nofollow noreferrer">brain fuel</a> is glucose, the long hard mental exam consumes most glucose available, and the result is poor cognitive function.</p>

<blockquote>
  <p>Glucose, a form of sugar, is the primary source of energy for every
  cell in the body. Because the brain is so rich in nerve cells, or
  neurons, it is the most energy-demanding organ, using one-half of all
  the sugar energy in the body.</p>
  
  <p>Brain functions such as thinking, memory, and learning are closely
  linked to glucose levels and how efficiently the brain uses this fuel
  source. If there isn’t enough glucose in the brain, for example,
  neurotransmitters, the brain’s chemical messengers, are not produced
  and communication between neurons breaks down. In addition,
  hypoglycemia, a common complication of diabetes caused by low glucose
  levels in the blood, can lead to loss of energy for brain function and
  is linked to poor attention and cognitive function.</p>
</blockquote>

<p>Mental effort drops glucose levels as reported by the study <a href="http://www.academia.edu/3991112/Mental_effort_blood_glucose_and_performance" rel="nofollow noreferrer">Mental effort, blood glucose and performance</a>, below an extract :</p>

<blockquote>
  <p>Work from our group and elsewhere has explored the link between blood
  glucose and cognitive performance.Here data are presented showing that
  a high level of mental effort is associated with a measurable drop in
  blood glucose levels. ‘Mental effort’ describes situations where energy
  is mobilised to meet cognitive goals. Reduced blood glucose has been
  reported in a number of such circumstances, including where
  computational demands of a task are relatively high (e.g. Serial
  Subtractions), during high processing loads (e.g. the Bakan task) and
  during response inhibition (e.g. the Stroop word-colour task).</p>
</blockquote>

CC BY-SA 3.0
2018-02-02T07:23:32.057Z
Can you provide a reference to an actual peer reviewed source? With an actual citation? Not a link to social media?
Bryan Krause
CC BY-SA 3.0
2018-02-02T07:57:53.107Z
The reference I added is from scientific journal   [Appetite Volume 47, Issue 2, Page 277 (September 2006)](https://www.sciencedirect.com/science/journal/01956663/47/2)
DesignerAnalyst
CC BY-SA 3.0
2018-02-02T15:33:10.797Z
It's just an abstract. It's not possible to review it's veracity from an abstract.
Bryan Krause
CC BY-SA 3.0
2018-02-02T15:39:48.770Z
Following up on the first author in Google scholar, [Andrew Scholey](https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=Andrew+Scholey&btnG=) does seem to return some full articles. Perhaps have a read of those (and related publications) to update your answer. In particular [this article, "Cognitive demand and blood glucose" seems promising](https://www.sciencedirect.com/science/article/pii/S0031938401004760).
Steven Jeuris
CC BY-SA 3.0
2018-02-02T16:55:29.053Z
Thanks @StevenJeuris; importantly, though, this study has not shown that the increased glucose metabolism during a task is due to increased brain usage of glucose, though I suppose this answer doesn't necessarily specify that either since it gives agency to "the long hard mental exam" as the consumer of glucose, rather than any particular physiological process. Additionally, the additional decrease in glucose they report is small compared to the magnitude of difference that they show causes slight cognitive effects on 1 out of 3 tasks (0.3-0.4 mmol/L versus 2 mmol/L).
Bryan Krause
CC BY-SA 3.0
2018-02-01T06:07:36.347Z
2018-02-02T05:58:03.977Z
2018-02-02T05:58:03.977Z
DesignerAnalyst
<h3>Question</h3>
<p>After taking a long final exam, people often report feeling like the world is moving in slow motion. I've heard this exam effect jokingly called &quot;zombie feeling&quot; or &quot;brain-fog&quot;. I describe the effect as strong desire to stare with a blank mind.</p>
<h3>Background</h3>
<p>Based on answers from this question: <a href="https://biology.stackexchange.com/q/839/238">https://biology.stackexchange.com/q/839/238</a> the amount of energy the brain uses doesn't really change when awake. So what could this effect be caused by?</p>
<p>Could it be caused by an exhaustion of certain nutrients, like Vitamin B12 or D?</p>

CC BY-SA 3.0
2018-01-31T21:38:00.340Z
If there's an exhaustion of anything, it's probably (blood) sugar.
got trolled too much this week
<cognitive-psychology><neuroscience>
What is an explaination for the light-headed and slow-motion feeling after a long hard mental exam?
Gabriel Fair
# What is an explaination for the light-headed and slow-motion feeling after a long hard mental exam?

(Asked by: username_0 on 2018-01-31)

### Question

After taking a long final exam, people often report feeling like the world is moving in slow motion. I've heard this exam effect jokingly called "zombie feeling" or "brain-fog". I describe the effect as strong desire to stare with a blank mind.

### Background

Based on answers from this question: <https://biology.stackexchange.com/q/839/238> the amount of energy the brain uses doesn't really change when awake. So what could this effect be caused by?

Could it be caused by an exhaustion of certain nutrients, like Vitamin B12 or D?

**Tags:** cognitive-psychology, neuroscience

> **Comments:**
> **username_1**: If there's an exhaustion of anything, it's probably (blood) sugar.  

---

## 1 answers

### Answer by username_2 on 2018-02-01. Score: 3

The [brain fuel](http://neuro.hms.harvard.edu/harvard-mahoney-neuroscience-institute/brain-newsletter/and-brain-series/sugar-and-brain) is glucose, the long hard mental exam consumes most glucose available, and the result is poor cognitive function.

> Glucose, a form of sugar, is the primary source of energy for every
> cell in the body. Because the brain is so rich in nerve cells, or
> neurons, it is the most energy-demanding organ, using one-half of all
> the sugar energy in the body.
>
> Brain functions such as thinking, memory, and learning are closely
> linked to glucose levels and how efficiently the brain uses this fuel
> source. If there isn’t enough glucose in the brain, for example,
> neurotransmitters, the brain’s chemical messengers, are not produced
> and communication between neurons breaks down. In addition,
> hypoglycemia, a common complication of diabetes caused by low glucose
> levels in the blood, can lead to loss of energy for brain function and
> is linked to poor attention and cognitive function.

Mental effort drops glucose levels as reported by the study [Mental effort, blood glucose and performance](http://www.academia.edu/3991112/Mental_effort_blood_glucose_and_performance), below an extract :

> Work from our group and elsewhere has explored the link between blood
> glucose and cognitive performance.Here data are presented showing that
> a high level of mental effort is associated with a measurable drop in
> blood glucose levels. ‘Mental effort’ describes situations where energy
> is mobilised to meet cognitive goals. Reduced blood glucose has been
> reported in a number of such circumstances, including where
> computational demands of a task are relatively high (e.g. Serial
> Subtractions), during high processing loads (e.g. the Bakan task) and
> during response inhibition (e.g. the Stroop word-colour task).

> **Comments:**
> **username_3**: Can you provide a reference to an actual peer reviewed source? With an actual citation? Not a link to social media?  
> **username_2**: The reference I added is from scientific journal   [Appetite Volume 47, Issue 2, Page 277 (September 2006)](https://www.sciencedirect.com/science/journal/01956663/47/2)  
> **username_3**: It's just an abstract. It's not possible to review it's veracity from an abstract.  
> **username_4**: Following up on the first author in Google scholar, [Andrew Scholey](https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=Andrew+Scholey&btnG=) does seem to return some full articles. Perhaps have a read of those (and related publications) to update your answer. In particular [this article, "Cognitive demand and blood glucose" seems promising](https://www.sciencedirect.com/science/article/pii/S0031938401004760).  
> **username_3**: Thanks @StevenJeuris; importantly, though, this study has not shown that the increased glucose metabolism during a task is due to increased brain usage of glucose, though I suppose this answer doesn't necessarily specify that either since it gives agency to "the long hard mental exam" as the consumer of glucose, rather than any particular physiological process. Additionally, the additional decrease in glucose they report is small compared to the magnitude of difference that they show causes slight cognitive effects on 1 out of 3 tasks (0.3-0.4 mmol/L versus 2 mmol/L).  

---

