My answer: 

There is a very sneaky sleep disruptor here that most parents wouldn’t ever be able to identify. Falling asleep fast (<15 minutes) at bedtime is not actually a good thing. When little ones are falling asleep really fast, it means they are already a good bit of the way down the road to sleep by the time they hit the mattress. Why does this matter? If they are, say, 60% drowsy when you put them down, they are only going 40% of the way to sleep on their own. So, when they wake up in the middle of the night, they need your help to get back to 60% drowsy, so they can take it from there. We really want them going like 90% of the way to sleep wherever they’re sleeping so that when they open their eyes in the middle of the night, they can say, “I’ve got this.” 🔆