I think it depends on the individual child. DD was 2.5 when we bought Alex home and their bedrooms were right next door to one another. She would sleep through anything, even when he was going though a phase of having night terrors that consisted of 45 mins of screaming. She is a fantastic little sleeper, I think you could drop a bomb on her and she would sleep through it! DS on the other hand is a light sleeper and is easily woken by noise from his sister, so it is her that has to tiptoe around when he is asleep. Oh well cant win em all!