The reason children tear open their Christmas presents in a frenzy of dawn excitement while grandparents leave theirs until after lunch comes down to how the ageing brain handles rewards. Scientists have discovered that a chemical in the brain governing the delivery and feeling of reward is altered physically as a person grows old, which explains why opening presents becomes less exciting.

When young people are involved in receiving prizes their brains become highly activated before and after being given them. This contrasts with the chemical activation in the brains of older people, said researchers at the US National Institute for Mental Health .

"Knowing how key brain circuits change as we get older may help us to rise to the public health challenge of ageing successfully," said Karen Berman, whose study is published in the journal Proceedings of the National Academy of Sciences.

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The results could also lead to new treatments for conditions caused by defects in the brain's "reward system", such as drug addiction and Parkinson's disease.