Why do basic yoga asanas cause me to have the most vivid and terrifying nightmares i have ever had in my life?

Yoga Dreaming: I am a psychiatrist w basic ( that means minimal) knowledge of yoga so the response I am providing is a theory only. As you already know, certain yoga poses can increase depth & lucidity of dreaming. My guess is that the content of your dreams is not directly related to the yoga but rather that your dreams are really delving deep & that the experience may be more intense & re- callable. Maybe >>.

Yoga & dreaming: Yoga allows you to access aspects of your physical experience that you might have ignored up until now -- and with it, possibly triggering sensations that are associated with fearful responses. Other possibilities that your yoga teacher may describe for you, are the idea of energetic awakening, called "kundalini" energy. This can be frightening for some people -- has been for some of my patients.