Why do people get grumpy when they're tired?

Answer 1

When you are tired, you do not have enough energy to do something which could bring sort of self satisfaction. You do not have enough energy to think what you can do. You need to have something done but you lack enough ability to fulfill such thing, thus unconsciously there is some sort of frustration which is unknowingly expressed as being irritable,highly sensitive and being grumpy.

Answer 2

I sure do and Grumpy hates it.Sleepy just yawns. And Doc prescribes rest.

Answer 3

You are serious, right? Generally, when people are tired, they tend to not have as much patience as when they are rested, revived and full of energy. It stands to reason that when someone is tired that they likely won't be in one of their best moods.