I nurse to sleep too, although sometimes, if she looks like she's not going to fall asleep, but she's tired, I can put her in her cot awake. Generally, I give her twenty minutes. If she's not asleep by that time I fetch her down to try again (before if she starts crying or is getting what I think is upset). I only usually have to do that once and she falls asleep the second time. It works for us, and she sleeps a good 12 hours (she's 23 weeks and 22 lbs) BUT I have to nurse her in our bed toget her down for a nap.