A few generations ago, when somebody said that the slept like a baby, what they meant was they slept all night long, without stirring, even if there were noises. Today, sleep like a baby often means wake up every few hours screaming. Something changed. What changed? The invention of the electric light and the invention of central heating and cooling. Because it turns out that the lights going extremely dim is a signal for your body to produce the hormones it needs for sleep. The same for falling temperature. Those are 2 of a number of different zeitgebers that can help sleep. If you've ever been camping, you know that when the lights get dark and it starts to get cooler, you get drowsy, drowsier far earlier than you would have otherwise, a couple of hours after sunset. So parents, wanting their kids to feel warm and safe, make the nursery cozy and warm and lit, which is doing exactly the opposite of what helps kids fall asleep. Having it be a little bit cooler, maybe 7 degrees cooler than it was during the day, and having it really, really dim, can help them sleep soundly, safely, securely, all night long.