In what ways do you behave, think or perceive differently when you're really, really sleepy?
[spoiler]As an example, some people might laugh a lot and very easily. Some might be more truthful than usual. Some might see everything as being brighter, or things might seem a lower pitch than they actually are. Some might start to think food tastes better when they're exhausted, or they might think everything in their life is perfect. Some people even get very sexually excited when they're on the verge of passing out.[/spoiler]
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I get grouchy and belligerent. If I get really tired (and I mean REALLY tired, like 48+ hours of no sleep tired) I start hearing a voice. Usually it asks "[i]Where are you?[/i]" and I start saying, "I'm here." Then it will usually say something like "[i]I can't find you. Where are you?[/i]" and I repeat my initial answer. After that I start talking to myself until I finally pass out. True story.