"A young child cannot yet understand what is best for him, and he may cry if he does not get what he wants," Ferber writes. "If he wanted to play with a sharp knife, you would not give it to him no matter how hard he cried, and you would not feel guilty or worry about psychological consequences. Poor sleep patterns are also harmful for your child and it is your job to correct them."
Proponents of these sleep training methods say it's okay for your child to cry when you put him to bed and leave the room, although they don't advocate letting a baby cry indefinitely. Typically, these methods suggest putting your baby to bed when he's still awake and allowing short periods of crying punctuated by comforting (but not picking up) your child.
Most sleep coaches say the ideal time to start sleep training (or promote independent sleep, not necessarily using the cry-it-out method) is based on your baby’s development, but is usually somewhere between four and six months, when your baby hasn’t had much time to get used to nursing or rocking to sleep. At this stage, most babies are also developmentally ready to learn the skill of falling asleep on their own, explains Jennifer Garden, an occupational therapist who runs Sleepdreams in Vancouver. Around four months of age, some babies go through a sleep regression because their sleep cycles change and there are longer periods of lighter sleep per cycle. “It’s a great time to work on independent sleep skills,” says McGinn. Other babies’ slumber derails around this time because they are working on new skills, like moving around and rolling. Some parents choose to wait until things settle down before embarking on a sleep-training method, but you don’t have to, says McGinn.