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.


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?

  • 2
    $\begingroup$ If there's an exhaustion of anything, it's probably (blood) sugar. $\endgroup$ Jan 31, 2018 at 21:38

1 Answer 1


The brain fuel 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, 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).

  • 1
    $\begingroup$ Can you provide a reference to an actual peer reviewed source? With an actual citation? Not a link to social media? $\endgroup$
    – Bryan Krause
    Feb 2, 2018 at 7:23
  • $\begingroup$ The reference I added is from scientific journal Appetite Volume 47, Issue 2, Page 277 (September 2006) $\endgroup$ Feb 2, 2018 at 7:57
  • 2
    $\begingroup$ It's just an abstract. It's not possible to review it's veracity from an abstract. $\endgroup$
    – Bryan Krause
    Feb 2, 2018 at 15:33
  • $\begingroup$ Following up on the first author in Google scholar, Andrew Scholey 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. $\endgroup$
    – Steven Jeuris
    Feb 2, 2018 at 15:39
  • 1
    $\begingroup$ 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). $\endgroup$
    – Bryan Krause
    Feb 2, 2018 at 16:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.