Small children suddenly cry when they fall asleep, may be related to hunger, temperature discomfort, frightened and other physiological factors, but also may be related to calcium deficiency, colds and other pathological factors. 1. Physiological factors: if a child is hungry when he sleeps, he can only express himself by crying because his language ability is not yet well developed; in addition, if the indoor temperature is too high or too low, or if he is frightened during the day, it will also lead to the above situation. 2. Pathological factors: small children if the body lacks calcium, may lead to crying, but also accompanied by occipital baldness and other phenomena, in addition to colds, nasal congestion can also lead to the above situation. The child suddenly cried in his sleep, there may be other reasons, it is recommended to go to the hospital in a timely manner to carry out relevant examinations, to clarify the cause of the disease and actively treat the symptoms.