During a nightmare your body muscles which are under your control (except for  eyeball muscles) such as legs, speech, arms, etc are paralyzed--ie you can't run  downstairs!! What she had was called a "night terror"--similar to what others do  when sleep walking. The person is asleep, may occasionally wake up  spontaneously, but until that time has no idea what is going on--night terrors  are really most frightening for the observer vs the patient. To learn more about  sleep issues pick up a copy of "Solve Your Child's Sleep Problems".