My DD woke up during the night at this age too - a friend suggested "night terrors" and i think that actually that's what it was. They are supposed to be different from "real" nightmares in that they don't remember them, and she grew out of it eventually (it took a month or so tho). Our DD didn't actually wake up tho - she used to cry in her sleep, and if we didn't wake her up properly, she would cry again in 5 mins time.