The Zi hour corresponds to the period from 11 PM to 1 AM.

Traditional Chinese medicine believes that after the Zi hour (from 11 PM to 1 AM, which is the time of the Gallbladder Meridian of Foot-Shaoyang), it is the time dominated by the Liver Meridian of Foot-Jueyin. This is when the liver dredges and regulates the qi movement throughout the body and when the body's yang qi begins to grow. During this time, the human body must enter a state of deep sleep. Deep sleep facilitates liver rest and supports the proper functioning of the liver's blood-storing role. As is known, the liver governs the body's dredging and dispersing functions. When these functions are normal, the various systems of the body operate more effectively, meaning the body can maintain order. Only with this order can the life cycle circulate and function properly.

Staying awake past midnight can impair the liver's function of dispersion and conveyance. When the liver fails to disperse and convey properly, symptoms such as dizziness, blurred vision, headaches, irritability, deafness, tinnitus, distension and pain in the chest and hypochondrium, palpitations and insomnia, and soreness and weakness in the lower back and knees may occur.

At this time, entering a state of deep sleep can facilitate the upward ascent of Yang Qi, promoting smooth circulation of Qi and blood. This greatly benefits the restoration and strengthening of the functions of various organs in the body.