Subject: Why do some dogs sometimes shake their heads vigorously after waking up?

We have two dogs.  Both of them exhibit a similar behavior just after they wake 
up- -  they shake their heads (and bodies) vigorously after waking up...  The 
move is basically identical to the move that dogs make when they try and dry off 
after getting wet.

I have also seen grizzly bears do this in Alaska...  Do other mammals do this?  
Why do any mammals do this?  Why don't humans do this?

Thanks, 

John Wilke, Greeley, Colorado.

Re: Why do some dogs sometimes shake their heads vigorously after waking up?