Why do i keep getting awaken from hypnic jerks while trying to "sleep off" a migraine attack? every time it was the same nightmare in which i free fell off a cliff, and every time i woke up with a racing heart and cold sweats. how do i stop it??

Sorry to hear that but you can try these things before going to sleep, an healthy social interaction, a warm shower, an empty bladder, avoiding oily food, alcohol, caffeine, nicotine, TV, and any other screens, drink a cup of warm milk if nothing is working then follow up with your PCP or a psychiatrist for pharmacological therapy or behavioral therapy. Good Luck.

These are likely "myoclonic jerks" which are normal in many when falling asleep. Vivid nitemares are a different story, and since this is emotionally upsetting, it might be of value to investigate further a sleep disorder such as sleep apnea, or narcolepsy. Sleep helps migraine, but perhaps better to prevent headaches by modern meds. See a neurologist.