Mirrors in fengshui are considered both good and bad. It is said that a person who receives a mirror as a gift will also receive bad luck. In addition, mirrors should never be placed in front of the bed because of the reflection that may cause you uncomfortable and distress.
Even though I have no experience of placing a mirror in front of my bed, but I had slept under a beam before. For the first few nights, I kept having nightmare and would wake up in the middle of the night. I started getting suspicious about the fengshui of the room and in the end; I found out that I should not place my bed under a beam. I changed the position of my bed and no more nightmares.
You may say that it is something superstitious; however, it is better to believe it rather than to ignore it.