Why don't kids get tired?

Why is it that when you are a kid, you don't get tired? it's time to go to bed, and you are just not tired. Now when you get to be my age (41), Woah! totally different story. I'm tired way before it's time to go to bed. I'm actually sitting with my kids and hubby watching TV, and trying super hard to not snooze away. I look at the clock as if it weren't on my side, "how in the world can it be 8:00 pm, if I'm already tired as heck". As we sit there together enjoying one of our favorite shows, I'm secretly thinking of that magical time..........when I go to my room and throw on some pajamas. Go to the bathroom - brush my teeth, wash my face, put my "anti-wrinkle" cream on, and creep into my lovable, cozy bed. That's one thing I absolutely love!! my bed. I find my bed so comforting, like snuggling in a big soft nest made of cushiony clouds. It's my personal sanctuary, a place were I find much needed rest and much needed love. hehehe. Sorry, I had to throw that in there. But back to my question..........why don't kids get tired? they go through their whole day, up and down, jumping, running.......no naps!. What magical powers do these little creatures have? I guess I was once one of them, and then I grew and life just happened.........and those little creatures sucked the living energy out of me! I still wouldn't have it any other way, and I will still find energy and rest in my precious bed, always.