There are several reasons why infants laugh in their sleep: imperfect development of the central nervous system, normal physiological activities, and disease factors such as epilepsy. 1. Imperfect development of the central nervous system: Because of the imperfect development of the central nervous system, infants during REM sleep. Facial muscles may show involuntary transient twitching, like a smile. This is an uncontrolled muscle response, which usually disappears with the gradual development of the central nervous system. 2. Normal physiological activities: Like adults, infants with normal development can also dream during sleep, resulting in facial smile movements. 3. Disease factors: Some diseases such as epilepsy can lead to facial muscle twitching in infants during sleep, showing upturning or non-stop twitching of the corners of the mouth, chewing of the corners of the mouth, etc. Usually, infants smile during sleep. Generally speaking, infants who smile during sleep usually do not need special treatment, but if there are abnormal phenomena such as twitching of the corners of the mouth, chewing of the corners of the mouth, it is recommended that parents take their babies to the hospital for medical treatment.