The occasional nightmare is considered a dream that’s simply more frightening or upsetting. Nightmares tend to be caused by stress, anxiety, or sometimes as a reaction to certain medications.
However, if you have nightmares frequently, you could have a sleeping disorder. Regularly occurring scary dreams can be labeled a sleeping disorder if the nightmares:

• cause you to be anxious about going to sleep
• lead to frequent disruptions of your sleep
• bring about other sleeping or psychological problems

Many people experience occasional nightmares throughout their lives. However, the American Sleep Association estimates only about 5 percent of the population experiences persistent nightmares as a sleeping disorder.

What influences dreams?

Some factors that affect us when we’re awake can also influence our dreams.
Health conditions
Foods
Daily activities

Some people, even some doctor advice for sleeping pills which can be a disaster for health. read more here