The reason why kids rip open their birthday gifts in a frenzy of dawn excitement, while adults leave theirs until after lunch has been found. According to researchers, it all comes down to how the ageing brain handles rewards.
Scientists at the US National Institute for Mental Health have discovered that a chemical in the brain governing the delivery and feeling of reward is altered physically as a person grows old.
The findings help to explain why it is that children find it almost impossible to contain their excitement on special occasions such as birthdays and Christmas, whereas the older we become the less excitable we tend to be.
The study has been published in the journal Proceedings of the National Academy of Sciences.
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.