What about bad dreams (nightmares)?.

 Although we have many remembrances to protect ourselves while we are sleeping, but sometimes we forget to recite it before sleep, and we may sees a dream; if it was a good dream we should thank Allah for it for our Prophet Muhammad (PBUH) said: if anyone of you sees a dream that he likes, then he should thank Allah for it and narrate it to others, and He said that a good dream (that comes true) of a righteous man is one of forty-six parts of prophetism. 

 When our Prophet said to his companions ''Nothing is left of the prophetism except Almubashirat, they asked : what are Almubashirat? He replied: the true good dreams that conveys glad tidings. And He said: when the day of Resurrection approaches; The dream of a believer will hardly fail to come true, and He said it used to be said: There are three types of dreams " the reflection of one's thoughts and experience one has during wakefulness, or what is suggested by Satan to frighten the dreamer, or glad tidings from Allah.   

 Satan can imitate any shape to frighten you except the shape of Muhammad peace be upon him for He said: whoever has seen me in a dream, then no doubt, he has seen me, for Satan cannot imitate My shape (cannot appear in My shape). 

But what to do if you sees a bad dream?. 

 The answer comes from the Messenger of Allah for He said: if anyone of you sees a bad dream, he should seek refuge with Allah from Satan and from its evil and should spit on the left (three times), and he should not mention it to anybody,  for the bad dream will not harm him.