What's wrong with a child crying at night?

Patient's question:

One and a half months
Detailed medical condition and consultation purpose: At night while sleeping, she cries suddenly about every half hour, stops crying as soon as she is fed, and falls asleep, but then starts crying again after a while; during the day, everything is normal, and she is very happy. This has been going on for some time. What could be the reason? Thank you!

Doctor's answer:

Hello: Difficulty for infants to fall asleep and crying at night – what can be done? This is a common issue faced by parents. Parents with similar problems report that their babies need to be held, rocked, swayed, walked, or suck on a pacifier to fall asleep. As soon as they are placed in bed after initially falling asleep, they wake up again, requiring prolonged holding, rocking, or pacifier use until they fall into a deep sleep before being put down. Such children often cry at night when they wake up, as they have developed poor sleep habits, making holding, rocking, swaying, or pacifier use essential as "sleep association conditions." At night, a baby with good sleep habits will automatically transition to the next sleep cycle when they wake between sleep phases, often without the parents even noticing. Infants who have developed "sleep association conditions" will demand that parents hold, rock, or sway them to fall back asleep when they wake. Even if they are not hungry, they will suck on a pacifier for a few moments before falling asleep, or they will cry endlessly otherwise.
How can parents cultivate the habit of their baby falling asleep on their own? The method is as follows: Starting from the daytime, place the baby in their crib when they are about to fall asleep. If they cry, comfort them with words and expressions at the bedside. If this doesn’t work, let the baby cry for a while before picking them up to comfort them. If they cry again after being put down, let them cry for a longer period before picking them up the second time. Gradually increase the time before picking them up in subsequent instances. Persist with this approach for 2–3 days until the baby realizes that even if they cry, their parents won’t pick them up, and they will learn to fall asleep on their own. The same method applies to breaking the pacifier habit. Once the habit of falling asleep independently is established, the baby will sleep well at night.
There are also other reasons for nighttime crying, such as overheating, being too cold, overeating, or hunger. Some infants are naturally more difficult to soothe from birth.