Being Tired

Why is it that a busy day makes us so tired? I really didn't expend that much more energy today (except for walking around more)... and yet I felt like I could go to bed as soon as I got home from work!

My life is about as easy peasy as life can get. I have a great job, great family and friends, a comfortable home... and yet sometimes I can get weary. I know everyone does... but it seems so silly to get tired when all I have to do is sit in front of a computer all day. I don't have to walk for miles to get clean water for my family. I don't have to grow my own food. In fact, I even pay someone to mow my lawn. I don't go to extra lengths to clean my house. So... what reason do I have to be tired?

None at all... and yet... it's off to bed for me!