The reason why your body suddenly shakes when you fall asleep may be related to being cold, or it may be caused by calcium deficiency or focal seizures. 1. Cold: If you don’t pay attention to keeping warm during sleep, your body will be stimulated by cold, which will lead to contraction of the skeletal muscles, and this may manifest as a sudden shaking while you are asleep. 2. Calcium deficiency: If the body is deficient in calcium, the concentration of calcium ions in the blood decreases, resulting in increased excitability of the muscle nerves, the body may show sudden shaking, which is common in the elderly, infants and young children, or pregnant women. 3. Focal epileptic seizures: If there is a history of epilepsy, focal epileptic seizures are likely to occur at night during sleep, which may manifest as a sudden shaking of the body during sleep, and may be accompanied by symptoms of twitching of the toes and ankles. Most of the sudden shaking during sleep is caused by physiologic factors, so there is no need to be overly concerned. If the symptoms occur frequently, it is recommended to seek medical attention as soon as possible to identify the cause and then treat the symptoms under the guidance of a doctor.