A month baby crying at night may be environmental discomfort, hunger, calcium deficiency, gastrointestinal discomfort and other reasons. 1. environmental discomfort: baby sleep at night in the process of environmental temperature is too high, the air is too dry, bedding clothing is too thick, too much noise, mosquito bites and other factors lead to the emergence of stuffy, itchy and other discomforts, so the night crying. 2. Hunger: more than a month of the baby’s metabolism is more vigorous than adults, rapid digestion and absorption, so the frequency of eating to be more, the baby at night may not feed for a long time and hunger, so crying. 3. Calcium deficiency: long-term lack of light leads to vitamin D deficiency, affecting calcium absorption of the baby, due to the lack of calcium ions lead to increased neural excitability, the baby appeared to be irritable, muscle twitching and other symptoms, so crying. 4. gastrointestinal discomfort: improper diet, abdominal cold and other factors lead to the baby’s gastrointestinal dysfunction, indigestion, so crying at night. One-month-old babies crying at night may be related to a variety of factors, it is recommended to consult a doctor for a clear diagnosis.