Perhaps check on any anxiety you may be feeling during your waking hours. School, family, work, S/O, health, money? Those are most of the cosmic issues.
Do you mean while you are sleeping? Are you sleeping during the day?
Probably you are stressed about something. Maybe worried about something upcoming? Or, feel you were treated badly by someone? What are the nightmares about? Are you getting physically hurt in them? Chased? Late for something?
@bkcunningham has a good point. Seems whenever I consume large amounts of sugar before bed, I not only have nightmares but just weird shit happens in them.
I find if I wear something that fits tightly while I sleep I have nightmares, particularly anything with a collar. To some extent, just being warm brings them on.
Maybe you have a lot of things in your waking life that you are just not ready to face yet, so your mind tries to sort through it while you’re sleeping.