I'm not posting this to start a debate about sleep training, although I'm sure it will kick off anyway. But I'm genuinely interested to know roughly what proportion of parents sleep train. By sleep train I mean any method that involves baby crying for any length of time, or leaving baby in cot for any length of time unhappily (including 'gentler' methods such as pick up put down). I had a discussion with a mum friend and we had very different estimations of how common it is.