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).