The first advice is to avoid it. Mirrors do not belong to the bedroom, especially not belong to the bed.
The only exception is  when the mirror is parallel to the headboard. (in this case you should not see the mirror when you are in bed). All mirrors stay in the bed room needs to be faced away from the bed. The test method is to lie in the bed and you should not see the surface of the mirror. 
If you are used to have a dresser next to the bed, you can put a piece of thick decorative cloth on the surface of the mirror and only remove it when you need to use the mirror. Cover the mirror can be very helpful if there is no better way to handle the situation. This applies to all reflective surfaces such as TV and computer screens. 
Cover the mirror is helpful only if it is covered with an object that can serve as a 'cover'. For example, 'cover with dust' does not count. A very genius bedroom design I have seen is to have remotely controlled doors for the mirror. You can also have a remotely controlled rotating stand for you TV. Remember, you spend one-third of your life time in bed for the purpose of resting. Make it a priority.