Zi Period means the period of the day from 11pm to 1am, and this period is the time when the Gall Bladder Channel 1 is in control. Being "in control" also means being "on duty".

There is a peculiar tendency that happens to people daily especially after dinner, at approximately 8or 9 p.m., when they start feeling sleepy. However, as 11p.m. arrives, people no longer feel drowsy and suddenly become very awake, and this is why many people have the habit of working (studying for exams, working late) after 11p.m. There are even some who start getting hunger pangs at around 11pm and they would proceed on searching the house for snacks. Why does this happen? It is exactly at this period when the Yang 2 in the gall bladder starts to develop, and this is why it is important to keep this principle in mind: go to bed before 11.p.m. This way, we can gradually develop better chances at prolonging and maintaining a healthier life because the relationship between sleeping well and longevity is very strong; therefore, sleeping better would develop a better Yang.

Zi Period is also the period of the day when it is the darkest and that is when the Yang starts developing according to TCM philosophy. In "Huangdi (a legendary ruler) Classic Dictionary", there is a sentence saying "The gall (system) controls the activities of the other 10 internal organs"; Hinging on the growth of the gall, the energy from the gall channel will develop too, thus, causing the body's vital energy and blood to rise together with the gall system. It is essential to one's day to develop good sleeping habits by going to bed during this period.