I personally think it is a number of contributing factors---the wiring of the baby, how the parents (mother) respond to them during the night (by this, I mean allowing them to nurse whenever they want for an extended period of time or setting some limits at night, and/or picking baby up andgiving them extra "stimulation" instead of letting them lay there and talking or patting or whatever), what the actual sucking need of the baby is and how that is handled (pacifier or breast), actual comfort of the baby (swaddling or not swaddling), growth spurts of baby.
You can't contribute ONLY AP or CIO for poor sleepers. I do happen to think, though, that if babies get used to having breast in their mouth all the time, that is what they get used to and CANNOT FALL ALSEEP WITHOUT IT. They get used to bedtimes and wake up times---or they get used to falling asleep and waking up whever they want. I think that a baby NEEDS to have some routines imposed instead of having them dictate their own schedule---their bodies are not capable of forming their own routine! And, when people say that babies are waking up and crying/screaming for the breast it is to "manipulate" the parents----I don't think that manipulate is the proper word here---but I DO BELIEVE that babies get used to what they have gotten on a regular basis and that is what they want, they don't have the ability to figure out any other way to fall asleep and stay asleep without having a nipple in their mouth! Babies are little people---and people get used to their own "routines" too----if you are used to having a cup of coffee when you wake up, then that is what you are used to and it is very difficult to NOT have that cup of coffee when you wake up and it is a hard habit to break. The same holds true for wanting the nipple in the baby's mouth to fall asleep---it is a HABIT that they have gotten used to, not something taht they are using to "manipulate" their mother with. Babies aren't capable of that!!
What I have read is that mothers who let their babies nurse all night and don't impose some sort of limits as time goes on end up miserable. You can have a child who is a poor sleeper or a good sleeper---but when they are totally addicted to the breast, that is where the most problems arise! Of course you want to comfort your baby and soothe them, and of course from the beginning that is the way to comfort them. But---I think mothers are setting themselves up for a lot of sleepless nights/months/years if they don't institute some limits and schedules as far as breastfeeding goes. They want to do the child-led weaning thing, and they give their child the breast whenever they want with no boundaries, and then when their child is 3 years old and ripping their top off at a wedding or other social event and screaming that they want boobies, Moms are at their end of their rope and crying that they need help.
I think that even though people say they do AP, they do it differently and that there are a lot of "improvisations" out there. I think you have to take it on a baby-to-baby basis----if you have a laid back and easy going baby, and if they sleep in good stretches then those improvisations don't have to be done. But, with a child that wakes up every half hour at night screeching for the breast and is unable to be put to sleep without hours of breastfeeding, then limits need to be imposed to preserve the sanity of mom and dad---or else they are on MDC pleading for help!! I think also that mom's view "limits" as doing a bad thing or "depriving" thier child---and that's not true at all. Certainly I am not talking about a 4 month old. But, when kids start to turn
1, then 2, and then 3 and up, and they won't sleep without waking up every hour to nurse and can't fall asleep on their own, that is where the problems really arise. And, unless you can sleep all night being chewed on, mothers end up majorly sleep deprived and miserable! Every mother wants their child to sleep, because sleep is important for development---but, mothers need sleep too, or else they go beserk!! You can't bend over backwards and deprive yourself of so much sleep just to get your child to sleep. You can certainly let your child know that they are not alone and that they don't have to be afraid without having them suck on your nipples all night long.
I think some of it also has to do with how much a child is stimulated when they wake up---if they get picked up, or they are just patted and talked to while still laying down, if they get nursed, if they are allowed to sit up and play, if the lights get turned on----all these things factor in, which is why I don't think it is easy to compare one with the other.
I know lots of people who were CIO babies, and they are fine (I'm one of them) I know people who have a terrible time sleeping and they were CIO babies. I know mothers who had their babies in their rooms/beds and they are great sleepers, and mothers who had their babies in their rooms/beds and they are horrible sleepers. There are A LOT of variables that come into play--HOW they co-sleep, are there 2 parents or one, are there siblings or no siblings, does light leak in the room at 5am or is it pitch black out at that time, is the baby a hungry type or not so hungry all the time, do they like cuddling or do they like their own space..........lots of variables. There are even big variables in the personalities of mothers/parents---you can have an extremely laid back mother who doesn't get all that upset with the crying and fussing, you can have a type-A mother who is affected by any little noise or "wrong" reaction of a baby, mothers who get mad quickly and mothers who don't have a temper.............
So---in my infinite wisdom
, there are genetic and environmental factors involved in sleeping, and comparing CIO with AP is difficult. Kids are different, adults are different, environments are different............I say whatever works for you is the winning method!!