A 3-week-old child crying in the middle of the night may be related to calcium deficiency, environmental factors, fright and other reasons, the specific analysis is as follows: 1. Calcium deficiency: If a child’s body lacks calcium and vitamin D, it may lead to crying in the middle of the night, as well as excessive sweating and occipital baldness. 2. Environmental factors: If the child’s sleep quality is poor due to the indoor environment being too high or too low, the child will also cry and cry, and the child will slowly return to normal after changing the environmental temperature. 3. Suffer from fright: If the child is frightened during the day, such as watching TV and seeing scary things or being intimidated by people or things around them, the above situation may occur. There may be other reasons for children to cry in the middle of the night, it is recommended to go to the hospital in time for relevant examinations, to clarify the cause of the disease, and then actively treat the symptoms.