I hate being that guy who asks where you infer it's a "small percentage", but my anecdata are that disrupted sleep is the norm. Yes, it got better after the first 3 months. Then there were multiple - very natural, I hear - sleep regressions: expansive brain growth which led to night talking/nightmares, teething, illnesses, fluctuations in natural body temperature regulation... kids are rapidly evolving creatures, and habits/pattern/training are great to instill, but there are a lot disruptors both natural and environmental.