Sleeping In

What time did you get up today? I got up at eight but hung around and ate breakfast and read a book and stuff before this thought came to my head. I know WHY people sleep in-they're tired or lazy. Simple as that. You slept at 2 AM? You won't be getting up for a while. You flew to USA from Japan? You'll be sleeping like a LOG. Or you are lazy and don't want to get up. That's what I don't get. I mean, there's nothing to do in bed! You just lie there! The only time I do that is when I randomly wake up at 3 AM and want to go back to sleep. But if it's a reasonable time to get up, like 8 AM, get up!!! There's a lot to do every day, and soem of that stuff needs to be done in the morning, right? Maybe I feel this way because I make a to do list every day I stay at home (without school) and list whether to do it in the AM, PM, or either. Hmmm...
Question of the Day: Why is sleeping in called, well, sleeping in? What does it mean by sleeping IN?