It is natural for a newborn to fall asleep while sucking at the
breast, a bottle, or a pacifier. When a baby always falls asleep this
way, he learns to associate sucking with falling asleep; over time,
he cannot fall asleep any other way. This is probably the most
natural, pleasant sleep association a baby can have. However, a
large percentage of parents who are struggling with older babies
who cannot fall asleep or stay asleep are fighting this powerful
association.