I never understood why my mom used to say that to me when I was a kid. I always thought "well, if you are tired why don't you go to bed"?
Now that I am the mom of teenagers I completely understand the sentiment. I am tired. I want peace and quiet. When you are awake I can't have that peace and quiet. I am also afraid to leave you unsupervised with friends while I am sleeping. I am tired and want to go to bed, thus you need to go to bed.
Long story short: Go to bed, I'm tired!
It's amazing that the older we get the more we realize just how smart our parents really were.