Why do people get grumpy when they are tired? Does it require more energy to be cheerful?
Some people say when you are tired, communicating with others is an effort. Your grumpiness discourages others from trying to be sociable. Conversely, if you are cheerful, they are more likely to talk to you. So it can be said that cheerfulness takes more energy.
Another theory is that you snap when you are tired, not because being grumpy is less energy-demanding than being nice, but because you are afraid that being nice will encourage people to chat and maybe ask you to do something.
Both theories suggest an evolutionary hypothesis for irritability and grumpiness: that they are an adaptation for communicating an urgent need to conserve energy.
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VICTOR TAM