There are three main reasons for this situation: the first is that during the day, the muscle activity of the legs is relatively large, resulting in muscle fatigue. The second is due to sleep at night, the legs have a cold situation, did not do a good job of strict warmth, resulting in blood circulation in the legs slowed down, and the muscles are in a relatively tense state, will aggravate the fatigue; and some metabolic products can not be effectively metabolized resulting in a buildup, will also appear to play soft, weak situation. The third is due to sleep at night, the leg muscles are not fully relaxed, such as sleep, leg muscles appear more tense, as well as sleep posture is not reasonable, resulting in fatigue of the legs. Therefore, in this case, the first thing you should do is to keep warm and avoid getting cold. During the night sleep, if you take the supine position, you can put a quilt under the bilateral knee joints, which can effectively keep the leg muscles relaxed and help recovery; if you are lying on your side, you can put a soft pillow between the bilateral knee joints to keep the lower limbs parallel, which can also effectively keep the muscles in a relaxed state.