21 months baby always wake up in the middle of the night may be caused by calcium deficiency, hunger, gastrointestinal dysfunction and other reasons. 1. Calcium deficiency: if often wake up in the middle of the night, and calcium deficiency has a certain relationship, when the lack of calcium, nerve excitability will be elevated, will lead to sleep disorders, easy to wake up at night. 2. Hunger: for example, infants do not eat enough, easy to lead to often wake up at night hungry, will lead to poor sleep quality, often wake up. 3. Gastrointestinal dysfunction: if the baby eats too much, it is easy to have bloating, gastrointestinal dysfunction and other symptoms, the body will be uncomfortable, it will wake up often at night. If the symptoms of the 21-month-old baby always waking up in the middle of the night continue without relief, it is recommended to consult a doctor in time to identify the cause and then treat the symptoms.