While many books suggest starting at 4-months (or earlier) the science (and your baby's development) may say otherwise.
This is a really interesting – and not necessarily straightforward – question. While a majority of sleep advice books warn parents that they should start training as early as they can (often 4-months or earlier), there’s virtually no research on the need to start this early. That being said, many parents have done this successfully and there’s no problem with that. Further, many parents need to be functional at work and/or mom is really in a sleep “emergency.” In these cases, parents really have to get things moving sleep-wise. HOWEVER, the question is whether you have to start this early---and the answer is firmly, no.
The vast majority of existing research on sleep training wasn't conducted on infants under 6-months.
Much of the existing sleep advice just plain scares parents with dire warnings that, "if they don't start early with establishing 'good habits,' their baby will never be a good sleeper." This is also not necessarily the case.
While it’s true that, if you do nothing, sleep problems may persist, research hasn’t ever investigated whether there’s any problem with waiting. In fact, most of the research on sleep training wasn’t even conducted on young infants. Even the strictest cry-it-out researchers wait until 6-months to include infants in their studies.
Why it's okay to wait
Limited self-soothing skills. Advice will tell parents that they need to let their baby cry at bedtime so that they learn self-soothing skills. What the advice typically leaves out is that babies are not born with a full repertoire of self-soothing skills. These develop as the baby’s motor and visual skills come online. At young ages, they only have a few tools in their self-calming toolbox. Claire Kopp, a developmental psychologist, suggests “levels of discomfort have to be relatively low in order for infants to be able to manage it on their own…Bringing thumb to mouth is done efficiently when an infant is awake and alert, but problematic when crying is intense.”
How much distress a baby can handle by themselves grows as they do. So, it’s important to go slowly in those early months and scaffold, rather than insist on, self-soothing skills.
The 4-month regression. The 4-month sleep regression is a time of massive brain and body growth. The brain is literally wiring itself for new visual, motor, and cognitive skills. They can’t just shut things out like they used to. A corresponding growth spurt may result in babies needing to feed more frequently. These events can make even previously good sleepers, wakeful and cranky.
Trying to sleep train at this chaotic time often results in more crying, and often less success, which can leave parents feeling frustrated and incompetent. Waiting until 6 months, allows for the baby skills to come online, for them to be able to better regulate their sleep, tolerate longer chunks of being awake, and have a more predictable nap and bedtime.
What if everything is going well?
If sleep is going well and the process is sustainable for you and for your baby, there's no need to really intervene. If you need to shift things later, there's no objective problem with that. The older a baby is, the more skills they will have. There might be things that are a little trickier, but your baby will also have more self-soothing skills and a better ability to perceive the routine that you’re trying to establish. Sleep training at older ages is definitely possible.
Do I have to use crying-it out?
For many babies and families, a “cry it out” approach is quick and relatively painless. If it fits with your parenting style and goals, then it’s a-okay. However, it may not work for many families or fit with their parenting style. There ARE choices when it comes to sleep training. Existing research and advice (and pediatricians) will make it sound like there's only one way. This is not at all the case.
Even though crying-it-out has been the most researched approach and works for many babies and many families, it is not the only way. Other approaches include gradual fading which involves the parents staying by the child while they go to sleep and gradually weaning down how much they do. Another approach involves just getting the timing of sleep correct for your baby's age. Often, just getting naps and bedtime timed well results in much better sleep. Parents can also choose just to work on skills at bedtime. That time is a time when parents are the most awake and the baby's circadian clock creates sleep pressure that helps facilitate sleep.
Whatever a baby has at bedtime is what they will need when they wake up at night. So, if you adjust bedtime behaviors, you may end up having better nighttime behaviors as well.
What you can do before you are ready to sleep train.
Naps, naps, naps. Make sure you know how long your baby can stay awake for their age. Prior to 6-months, it is okay to get naps in any way that works. According to Kim West, The Sleep Lady, these can be easily shifted to crib sleep once the regression is over. A well-napped baby will have an easier bedtime and fewer wake-ups.
Test the waters. Up until 6-months, it’s okay to experiment with seeing if, after feeding, you can put them down really drowsy and they will go the rest of the way to sleep on their own. If not, no worries. Keep practicing. You are at least laying a foundation for later sleep skills.
Rule out physiological sleep busters. If your baby has difficulty sleeping anywhere, anytime, make sure you don’t see signs of silent reflux: arching, pain when lying flat, sleeping better upright, waking up screaming; or obstructed breathing: snoring/breathing through the mouth (not associated with a cold) or a sweaty head while sleeping (not associated with a warm sleep environment). These are things that interfere with sleep and should be checked out.
There really is time to work on sleep and it’s okay to make the choice that works for your family. Take a breath, parents. It’s a long road. Pace yourself.