Mystique is a metamorph in the X-Men franchise.
When she imitates another person, does she need to actively maintain her transformation? Does she need to concentrate or at least think of being like the person she imitates, in order not to turn back to her natural form?
Furthermore, if she can transform into anyone and stay so without any effort - e.g. even while asleep or unconscious - then how does she even know what her natural form is?
Historically, she has reverted to her natural form when she has been rendered unconscious, or when she's suffered severe trauma (this may have changed recently, I haven't kept up with the comics).
She does seem to be able to maintain her form while sleeping.
This leads me to believe that her shapeshifting is less like tensing a muscle or concentrating and more like breathing or holding your bowels - it does require some degree of focus, but she has mastered that focus so completely that she maintains it reflexively.
This IS something that can be trained and learned (as any parent can tell you) and it's only reasonable that she would have enough practice to be able to do it.
