Anyone who has laboured long and hard over a tricky crossword puzzle knows it’s true.

Now scientists have come up with proof that too much thinking can be exhausting. Older people, in particular, are likely to notice the impact of straining the grey matter, the experts say.

They have found that concentrating hard on a task drains glucose from a key part of the brain.

Glucose, which is transported around the body in blood, is the brain’s main power source.

The findings suggest a small piece of chocolate or a glucose sweet would help get energy to the brain before a crucial exam or test.

The scientific team, from the University of Illinois, carried out their research on rats. When the rodents had to concentrate on finding their way through a maze, glucose levels in brain cells connected with orientation dipped by almost a third.

In older rats, levels dropped by half, producing a big deficit in performance, and recovery took 30 minutes.

But in younger rats levels fell by only 12 per cent and recovered quickly. Glucose injections boosted the animals’ performances.

Professor Paul Gold, who led the research, published in the science journal Neurobiology of Learning and Memory, said the findings might have important implications for timing of meals in schools and colleges and could aid understanding of age-related memory and learning problems.

Dr Ewan McNay of Yale University, another member of the team, said: ‘A lack of fuel affects the ability to think and remember.’

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