Every trouble in a child's life is the result of something. Sometimes, simple unawareness in the childish psychology could cause many uncured psychological complexes and troubles. Every children psychologist suggests that it is better to prevent than to cure.
Nightmares
The nightmares are well-known guides of the kids. Imagination plays bad role this time. Check what your child read, watch, play or observe during the daytime. Perhaps, he/she is reading the scary fairy-tales or play Starcraft for two hours per day, or he/she suffers of the family problems (divorce, quarreling, dissatisfaction, punishment, pet death, friends' trouble and so on).
Some children are afraid to stay alone without light and keep screaming or crying when you leave the room. Make sure that there are not any imaginary monster somewhere at the room and calm down your child. Encourage a child to wait for a while in order to relax and think why he/she is so anxious. If he/she does not stop screaming, make the short visits to the room to show that you are not ignorant about him/her. But the visits are to be shorter, the periods between them are to become longer until the child calms down and sleeps.
In fact, children's fears could trace into an adult life. Be attentive to your child's growing and try not to miss the time when the things are possible to warn beforehand. The positive idea is that it is possible to teach a child to control himself if he feels fear about something and avoid many phobias in the future.