1 year old won't sleep through anymore

[hopingtoo]

Hi - my 22 month dd was a terrible sleeper for first year but for last few months has been sleeping through from 7.30ish, albeit waking around 5.30/6. Suddenly, last week, she has started waking in the night - around 1/2am - and refusing to go back to sleep. She shares a room with 5 year old sister - great sleeper - so when she wakes we take her into our room where we still have a cot so as not to disturb. When she wakes around 5.30 we have been taking her into bed with us and sometimes she sleeps. Now she expects to come in with us at 1am - and we aren't playing ball. We are being reassuring but firm, laying her in cot, calming her, going back to bed - but she has begun to launch herself out of her cot (it's on the lowest level) and last night it took nearly 2 hours to settle her. I expected it with a newborn but now I'm back at work and just can't cope with this - advice welcome!