Sudden loud crying at night may be caused by physiological factors, diseases and other factors. 1. Physiological factors: such as being frightened, excessive hunger, not changing the diaper caused by the baby discomfort, and other physiological factors, will lead to infants suddenly crying loudly at night, parents should promptly find the cause, eliminate the triggers will return to normal. 2. Vitamin D deficiency: Vitamin D deficient babies can appear, occipital baldness, night terrors, excessive sweating and other manifestations, you can improve the blood activation of vitamin D level check, under the guidance of the doctor to supplement vitamin D treatment. Babies suddenly cry loudly at night if they can not be pacified to relieve or accompanied by other abnormalities, need to be alert to disease factors, should also be timely medical examination and treatment.