I have bad dreams once in awhile and tend to wake up stiff, achy and exhausted after a nightmare or particularly vivid dream. why is that?

Once a week or more?: Hi, how often is "once in awhile?" Occasional nightmares, though disruptive, are are not unusual. As to why, multiple things can trigger bad dreams, stress, alcohol, medications, and sleep disorders. Won't hurt to check in with your MD to see if you have other symptoms that warrant a sleep study. Therapy (e.g., rescripting) and/or medications can be helpful if the problem persists. Best wishes