If they appear in your dreams it is possible that it is caused by, for example, improper sleep hygiene, sleep deprivation, traumatic experiences, heavy foods eaten before bed, excess stimuli before going to sleep, etc.
Take care of your sleep hygiene. Eat an easily digestible dinner, do something before bed that makes you relaxed and calm such as meditation, listening to music or reading a book. Also avoid using the phone or watching TV before going to sleep.