So WHAT is the source of this belief?
Well, some people say a mirror facing the bed is “Bad Feng Shui.” In Feng Shui, a Chinese system considered to govern the flow of energy (Chi) through the spatial arrangement and orientation in a room or home. An uncovered mirror can bounce the Chi around the bedroom at night, which may result in restlessness. Apparently, bouncing Chi disturbs a good night’s sleep.
Prior to Feng Shui becoming big in the USA, there seems to have been a somewhat obscure American folk belief that maintains that a mirror should not be hung in a bed room so that a person in the bed might spy her/his reflection if waking up briefly at night. This idea likely originated in the British Isles.
Specifically, when someone is lying in bed she or he should not be able to see into the mirror either from:
If you sleep with a mirror at the end of your bed, bad things can happen. What exactly happens is a tad unclear. However, in American folklore, ghosts, specters, or other ominous visions may be seen in a mirror in a darkened room.
The source of this belief may be an 18th century type divination in the British Isles using a mirror (Latin: catoptromancy). A maiden should take a candle and go alone to a looking glass; eat an apple before it, and some traditions say, you should comb your hair all the time; the face of her husband to be, will be seen in the glass, as if peeping over your shoulder. Yet, woe betides the maiden who sees a skeleton or coffin behind her--for she will die unmarried.
The above are the best explanations I have of why some people warn against sleeping with a mirror facing the bed.