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?