my daughter is 18 months and until now has been a text book child when going to bed getting herself to sleep without us being in the room. Overnight she has become hysterical & genuinely distraught if we leave the room before she's fallen asleep and also wakes up at night unable to fall back asleep, so we repeat the long process of getting her to sleep or eventually bring her to bed with us out of sheer exhaustion and desperation. Any idea why the sudden change and any advice on how to deal with it?