It was the afternoon of a week long, 8 hour each day course on well known (but hard to interpret) European authors.  The instructor, looking somewhat bleary-eyed herself, asked the seminar students to comment on the philosophical context of a story just read, and the silence that greeted her motivated a break. “I feel brain dead, “was heard during the exodus to the cafeteria for caffeine and snacks.

Fading cognitive performance, defined by the inability to concentrate, think clearly, recall information or generate new ideas may happen at any time but, according to much psychological research, it is most likely to occur in the afternoon as my class exemplified. Two hours earlier, comments and analyses were coming quickly and generating active discussions. Now there was only silence. 

The tiredness that overcame the class was in a sense like the muscle fatigue occurring after a hard aerobic workout or marathon.  There was very little reserve mental energy.

This being a classroom, not a courtroom, nor operating room, nor cockpit of an airplane; the deterioration of brain power was not going to affect lives or legal outcomes. But even in a classroom, the failure of the brain to maintain higher productivity from earlier in the day has been noted among students taking standard exams over several hours. In a recent Danish study, Sievertsen and his colleagues found a measurable decline in test scores for every hour that passed in the day. The scores worsened significantly in the late afternoon. (AndC C

This is not a new complaint.  Much earlier study giving volunteers tasks that measured cognition also found a decline in cognitive performance and attentiveness with time of day.    No one has yet suggested flying only in the morning with a still mentally fresh pilot, or scheduling a surgical procedure before 3pm. But a friend who had to have his cataracts removed insisted on the procedure being done before noon.

Is there a way to reverse this mid afternoon brain fog? A classroom of students was dedicated to seeking an answer. Attention span and creative thinking, or at least the ability to figure out the hidden meanings in the stories, was restored by most of the class in about twenty minutes. Students consumed caffeinated beverages, carbohydrate snacks (mostly low-fat crackers and some leftover bagel halves from lunch), and most had taken a quick walk outside, or sat in the sun. Curiously, no one had advised them on how to stop their brain fade, but they all ended up doing pretty much the same thing and it worked.

Caffeine has been singled out as probably the most potent restorer of cognitive function and focus. Its effect is easily felt after many hours of non-caffeine intake when the first cup of coffee in the morning is consumed.  But additional caffeine consumed after several cups of coffee or the equivalent in a test solution can still enhance performance on cognitive tests. Caffeine blocks the action of a particular chemical in the brain, and allows instead for dopamine and other alertness brain chemicals to become more active. The effect is to perceive oneself becoming more alert with less brain fade. Of course, ingesting caffeine in the afternoon has its risks as well, mainly this of insomnia several hours later.

So other brain fixes have to be used if this is a problem.

Eating a starchy, low or non-fat carbohydrate works to reverse brain fatigue, but not as a stimulant. Rather, the carbohydrate seems to maintain and/or restore focus.  Studies were carried out with the military to see whether consuming carbohydrate maintained vigilance and attention on difficult cognitive tests even after the soldiers were exhausted from prolonged strenuous exercise. The volunteers were given carbohydrate or placebo in liquid form after hours of forced marches, and then the scientists measured their cognitive performance. The testing situation was supposed to determine how solders could maintain a high level of brain work even when physically spent.  Those getting placebo did the worst on the tests; those consuming the carbohydrate did significantly better.

The positive effect of consuming the carbohydrate was due to the brain receiving its crucial energy supply of glucose, and also because serotonin levels increased.  This neurotransmitter, which is made only after eating carbohydrate, had been shown decades earlier to filter out distraction and improve concentration.

But some of my classmates in my literature course opted for a carbohydrate treat instead of “treatment” and returned from a visit to a vending machine with candy bars or chips. While they were eating carbs, the fat in these snacks slowed digestion considerably, and they may not have felt the positive effects of the carbohydrate for more than an hour.  Fortunately, no one was on either a carbohydrate-free or extremely low-carbohydrate diet. If they were, they would have no way of combatting their mental tiredness. Ironically, the afternoon slump has long been seen as a response to a high-fat lunch regardless of brain work. One wonders how the brain recovers from this slump, and additional exhausting brain work if denied carbohydrate. While it is true that after several weeks of a totally carbohydrate-free diet the brain adapts and uses fat for energy rather than glucose, during the period of adaptation, a phenomenon known as brain fog occurs.

This is something else to consider when booking that surgery.  


“Cognitive fatigue influences students’ performance on standardized tests,” Sievertsen, H., Gino, F. and Piovesan, M.,  Proc  Natl Academy Sci U.S.A., 2016; 113(10): 2621–2624.

“Diurnal Variation, Task Characteristics, and Vigilance Performance,” Craig, A, Davies R. Matthews,G., , J of Human Factors and Ergonomics Society, 29; 675-684, 1987.

“Acute caffeine consumption enhances the executive control of visual attention in habitual consumers,” Brunye, T., Mahoney, C., Lieberman, H., Giles, G., and Taylor H., Brain and Cognition 2010; 74: 186-192.

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