Cramps appear in the middle of the night, the reasons are usually the following: First, during the day, the amount of activity is relatively large, the legs and other locations of the limbs of muscle tissue overworked to form strain, resulting in inflammation, lactic acid and other metabolic products accumulation, during the night sleep is easy to produce stimulation of muscle tissue, inducing muscle cramps. Second, the cold at night, especially the daytime activity is relatively large, after the cold at night muscle tissue becomes more tense, blood circulation will also decline, metabolites more and more easily on the muscle stimulation, inducing cramps. Third, when sleeping in the middle of the night, if the bedding is too thick and heavy, the whole ankle joint is in a state of plantarflexion, and after a long time of plantarflexion the muscle tissue is overworked, which can easily induce cramps.