It must depend on the child. My DD was 4 1/2 when we brought the new baby home and she was fine at night for the night feeds and stuff. where it did and still does affect her, is early mornings. She is an early riser at the best of times, and she is up the instant she hears DH or I in the mornings, so same thing with Sebastian. If he cries/wakes up after 5am, then the WHOLE HOUSE is up at 5am (eeergh!). Anyway, their rooms are right next to each other (ie they share a wall) and she was fine at night. Good luck!