Who’s bed is it anyway?

When I go to bed I make myself really comfortable, and stretch out in just the right position, while I read for a while.
But by, say, 4am this is the scene.  Portia has arrived and occupies the centre, making herself truly comfy.  I seem, without ever waking up, to have been restricted to a small diagonal space over in one corner, with my feet sticking out entirely.  
But how does this happen, night after night? I think perhaps cats are determined to get exactly what they want, and humans often concerned not to be in the way, or inconvenience others. Even when they’re asleep:)