《黄帝内经·灵枢·大惑论》
卫气在人体循行的常规是白天行在阳分，夜间行于阴分，当卫气随昼夜交替在人体阳分运行已尽，由阳入阴时，人就入睡；卫气在人体阴分运行已尽，由阴出阳，人便醒来。
夫卫气者，昼日常行于阳，夜行于阴，故阳气尽则卧，阴气尽则寤。
The regular pattern of Wei Qi's circulation in the body is: during the day, it flows in the Yang portion; during the night, it flows in the Yin portion. When Wei Qi finishes flowing in the body's Yang portion as the day and night shift, and flows from Yang to Yin, people will fall asleep. When Wei Qi finishes flowing in the body's Yin portion and flows from Yin to Yang, people will wake up.