I'm using a dummy just for sleeps and when we are out n about, so far no probs (besides bub losing it during sleep and wanting it on occasions). The nurses at the 'sleep school' I took bubs to (to sort out her day sleeps) were all for either thumbs or dummies with the proviso that they only use dummy and/or thumb for sleeps. In fact, when they put my bub down to watch her sleep it was without a dummy - they were hoping she'd find her thumb and therefore "self-settle" - she didn't but we try a few times at each sleep with no dummy 3-4 times and then resort to giving it to her but we give her the opportunity not to take the dummy - she doesn't want a dummy when she wakes for a night feed - she goes straight to sleep without thumb or dumb then.