You know you're tired when...

You can't bring yourself to put on your pajamas, even though they're right next to you.

You don't care how you look. Screw the half eyebrow on your face.

You can't be bothered to get up to turn off the light when the switch is two feet away.

You start laughing at things that aren't really funny. "HAHAHA! Look! Look at that person's status! I don't get it! Haha."

You realize your still up writing about this instead of doing all of the stuff above and going to sleep.