Let's say you're a baby. You reach the end of a sleep cycle and become partially alert. What do you do? Ideally, if nothing were wrong you would settle back in for another sleep cycle. Chances are, however, you don't know how to do this yet, so you wake up more fully and start to cry. This seems to reliably trigger some combination of cuddles, shushing, motion, and/or nursing—comforted, you fall back asleep. While this is a fine outcome for you, your parents would probably be doing a lot better if you didn't rely on them so heavily at night.

With a very little baby this is unavoidable, though it can be partially automated, but as babies get older they need to learn how to handle this themselves. Sleep training is a collection of strategies for teaching babies (a) the skill of falling asleep on their own ("self soothing") and (b) when they should apply this new skill.

One of the things that makes discussion of sleep training messy is that people often aren't clear on which of (a) or (b) they're talking about. For example, a strategy of putting your kid down when they're very drowsy but not quite asleep yet is about teaching (a), while applying consistent patterns about when or how often you'll feed or settle them is about (b). If you think someone is trying to do (a) when they're actually doing (b), their approach can often sound excessive, uncaring, or unfair.

It can also be hard to tell which of (a) or (b) your child needs. A child who can self-soothe but prefers to be cuddled doesn't appear that different from one who doesn't know how do it yet. Gradually providing less and less comfort can work well in these cases, because it's a good way to teach (a) and a so-so way of teaching (b). Alternatively, cry-it-out is a decent option for (b) and can also be a way of teaching (a) if other options haven't worked.

With our youngest we found that tapering with the Snoo taught (a) reasonably well: while there was some crying she mostly was able to gradually build up the skill. With our oldest, pre-Snoo, we tried various methods of trying to teach (a), but it wasn't until we switched to cry-it-out that she really mastered the skill, and started taking longer naps.

Our youngest still tests the boundaries of (b) sometimes. For example, two months ago (age 13m) we were on vacation (28 people in a 5br). She quickly figured out that every time she cried at night she quickly got to nurse: we didn't want her to wake others up. Over the course of the week she started crying more and more often during the night, correctly (and unfortunately) learning that the adult-implemented pattern for when she needed to go back to sleep on her own had shifted. After we got back home we had about a week of gradually re-teaching her the normal pattern.

Another consideration I think is often missing in discussions of sleep training is that while crying is unpleasant for the baby, better-slept adults are able to be better parents in other ways. I think the baby typically is better off, even in the short term, with a couple weeks of cry-it-out vs months of zombie parents.

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We were unhappy with the cry-it-out method and rolled our own step-by-step approach. Step by step is often a good idea because it keeps the child in its Zone of Proximal Development, and that avoids local optima and allows the child to really get it. It requires more work from the parents, and I agree with Jeff that zombie parents probably can't do that.   

I agree all around, great discussion.

We did cry-it-out sleep training following the instructions in Ferber, which also has really great explanations of what you’re trying to do and how and why.

Why did babies evolve to wake so frequently at night? Did frequent night waking have some evolutionary benefit to babies that outweighed the risks of sleep deprived parents?

I don’t know if there’s been much research into the long term effects of sleep training? The best information that I could find was this article https://www.bbc.com/future/article/20220322-how-sleep-training-affects-babies.

The Snoo has got to have one of the catchiest product names out there. Every time I see it in print, I have the urge to call out "the Snooooooo!"