What is an explaination for the light-headed and slow-motion feeling after a long hard mental exam? 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 &quot;zombie feeling&quot; or &quot;brain-fog&quot;. 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?
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).