According to Feng Shui principles, one should never sleep by a mirror, and also, a mirror should never be located opposite the bed.
From a psychological point of view, it is quite understandable because men and animals have peripheral vision, which is used to warn of possible danger. The reaction to the threat detected by peripheral vision, is triggered by instinct, not knowledge. Animals flee or attack if something threatening enters their peripheral vision.
In humans, this is much more complicated, because a collision occurs between consciousness and instincts. So you can feel constantly nervous without realizing why. This is due to the fact that in your peripheral vision, your own reflection is entering.
That happens without you realizing it and may cause a state of causeless depression. At night, the mind loses control of the subconscious sleep state when you see your reflection, it will work poorly on your psyche.
For the same reason, do not hang a mirror where you constantly sit, like in the living room and kitchen. Perfect place for a mirror is the corridor.
In the past, people were convinced that during sleep the soul left the body and went into the mirror, facing the bed, thus one will die because his soul can not leave the mirror.
It is believed that the mirror is a gateway to another world, and that is why mirrors are used in many magical rituals. It is known fact that after the death of a man, one always puts material over the mirrors in the house.
According to Feng Shui, mirrors hung on the bed change the flow of life energy and thence to a bad start.
It is good to put a mirror on the outside of the door to the bathroom or toilet, as according to Feng Shui, your luck will not runs out with the water.