Thinking hard is just as tiring as manual labour.
Thinking hard is as exhausting as physical labour
Experts have revealed that desk jobs are as exhausting for the human brain and body as physical work.
Scientists explained that long periods of concentration cause toxic chemicals to build up in the brain and means it must power down to protect itself - making us feel tiredness.
People who rack their brains at work showed the same signs of weariness as if they were physically exhausted, including smaller pupils and making lazy choices.
For the study, 40 people spent six hours on puzzles and those who completed hard tasks made easier decisions at the end of the day - choosing a smaller, faster reward as opposed to waiting for a bigger prize.
Tests also showed a bigger build-up of brain molecules called glutamate in those who completed the difficult puzzles. These are important for brain function but are dangerous in large amounts, meaning the brain must work less if levels get too high.
Brain expert Dr. Mathias Pessiglione said: "Fatigue would be a signal that makes us stop working to preserve brain function.
"Glutamate regulation has been pointed out as an essential component in the brain's energy budget and is a potential source of cognitive fatigue."