How does sleep training work?

(A baby uninterested in sleep.)

About two months ago, M began waking up even more often in the middle of the night and it became hard to get a decent night's sleep. (Things had never been great, but they got noticeably worse). We decided to sleep train. Sleep training for infants is a very emotionally-charged topic for parents. Along with its strong advocates, you can find people arguing that sleep training is child abuse.

M's mom and I found a meta-analysis suggesting the efficacy of a variety of sleep-training techniques and a strong recent study that found no major negative effects. In addition, many of the critiques seemed like misreadings of the literature on infant attachment. So we decided to go for it. Based on a survey of friends, blogs, and our own conscience, we selected the "graduated extinction" method – also known as "Ferberization" – where you let the baby cry for gradually longer intervals before providing comfort. (I found Ferber's book to be the clearest and best-written of the baby sleep guides, as well).

The process was fairly traumatic for us. But it was comforting to see that M showed no evidence of caring in the morning – she still seemed to like us just fine. And after a few nights, she went to bed without crying; after a few more she slept through the night without waking. This amazing result has continued (with a few interruptions) for about a month and a half. It's an exaggeration to say that either of us is well-rested, but the situation is markedly improved.

Now on the other side of this process, it's easy for me to say that – despite many stories of cases where sleep training doesn't work well for individual kids – the evidence for population-level efficacy seems pretty clear. What I found astonishing, though, is how little discussion there is of why it works. In addition, the explanation that is on offer – Ferber's own – doesn't make much sense. It uses the language of behaviorism, but on a closer examination, it isn't consistent with actual behaviorism at all.

Ferber gives the following preamble to his instructions:

The goal of this approach is to help your child learn a new and more appropriate set of associations with falling asleep so that when he wakes in the middle of the night he will find himself still in the same conditions that were present at bedtime, conditions that he already is used to falling asleep under. But, to do this, you must first identify the pattern of associations that is currently interfering with his sleep (and yours) and which he must unlearn.

This seems straightforward: the child has associations with falling asleep – being rocked, maybe having a bottle, perhaps lying upright on your shoulder – that prevent him or her from falling asleep in the crib. You break these associations and the child learns to "self-soothe" and put him- or herself to sleep.

This analysis sounds reasonable. But fundamentally, its reasonableness is post hoc, like a lot of other psychological explanations. The technique works, so we accept the explanation. But there are many similar explanations that we would accept if they were linked to a technique that worked. And if the technique didn't work, we'd easily discard the explanation.

A big part of the prima facie reasonableness of the Ferber explanation is that it's couched in language we know: the associative language of classical (Pavlovian) conditioning. In classical conditioning,  a tone (conditioned stimulus: CS) is paired with a shock (unconditioned stimulus: US) to induce a fear response (conditioned response: CR). Eventually, the tone produces the fear response without the shock. (Somewhat) similarly, cuddling is associated with sleep. Eventually, cuddling becomes prerequisite for sleep. To allow for sleep without cuddling, this association must be "extinguished."

But there are a number of places where this analysis breaks down:

• What's the US (the shock) in this case? Is it drowsiness? Presumably baby still needs to be drowsy to fall asleep after cuddling - though the cuddling might help the process along. So the US isn't ever out of the loop here.
• Sleep isn't a stimulus-triggered aversive behavior like fear-based freezing in rats. Drowsiness is a feeling that is generated by the baby herself, more like hunger or thirst. I'm definitely not an expert on classical conditioning, but a quick reading of the literature suggests that maybe it's not quite so easy to condition appetites. 
• Conditioning of a response to some cue doesn't mean that the response cannot be triggered by another CS (though there maybe some weakening), and especially doesn't mean that the US is no longer powerful. For example, a rat conditioned to freeze based on a tone will still freeze based on a shock. So why on this analysis can't babies sleep without "sleep associations"? Presumably they still get drowsy.
• Finally, nothing anywhere in the conditioning analysis predicts crying as an outcome of not having the cues for sleep. I guess the idea is that the baby wants to sleep, but doesn't have the cues to allow sleep, and then is frustrated? But that extension of the explanation definitely isn't congruent with a conditioning analysis. Instead it reflects some kind of second-order response: frustration about the mismatch between the baby's needs and what's happening (I want to sleep but I can't). 

So despite the superficial theoretical trappings of behaviorism, it doesn't seem like a standard conditioning analysis predicts many of the salient parts of the phenomenon. 

Perhaps I haven't lined up the pieces right here. Perhaps there is actually a classical conditioning analysis that makes sense, but my still-somewhat-sleep-deprived mind hasn't been able to work it out. If so, I'd love to hear it. In the absence of that kind of analysis, though, the overall success of the Ferber method is even more magical.