Why is it...

Why is it that when you're really tired and all settled in to sleep, your brain starts racing through the long list of things that need to be done upon waking the next day, totally keeping you from actually falling asleep? Why haven't they invented an on and off switch for just such an occasion? Or a mute button for your children? Or wouldn't it be nice if you could hit an "undo" button for those little mistakes. Scientists come up with lots of cool helpful things, why can't they get a little more practical and solve the everyday aggrevations that keep us from remaining sane? Is it just TOO much to ask?