The device needs to be able to survive dropping to the floor repeatedly.
I am one of those guys that fall asleep with a book in their hands. The book then either drops on my forehead, or on the floor (depending on the position I was last in before dozing off :P )
On a sidenote; the former (dropping on my forehead) made me stop buying hardcover books. The last hardcover book I bought was Robert Jordan's 'A Knife of Dreams'. 780 pages covered in rock-hard paperback dropping on the nugging do hurt ... I can tell you
