Early to bed and late to get up 

In ancient times, there was low productivity, and people were not busy with farming in winter, nor did they have enough measures to protect against the cold. What they could do was just passively adapt to the climate change, and stay in their home similar to the hibernation of other animals. Therefore, their work and rest in daily life had to be adjusted in accordance with nature.

When it comes to the way of preserving health in winter, early to bed and late to get up are advisable: early sleep is to replenish the yang qi in human body and keep warm of the body; getting up late is to nourish the yin qi, and beginning work at sunrise is beneficial for avoiding the cold and keep the body warm, so that the yin is calm and yang is sound. Furthermore, it can prevent low temperature and cold air from invading the human body, triggering respiratory diseases and cardiovascular and cerebrovascular diseases.

Of course, “late to get up” means that it is not necessary to wake up before dawn after the long night in winter, rather than actually sleeping late into the morning. Adequate sleep is beneficial to recover the strength and improve the immune system, which can help to prevent diseases. However, if the sleep time is too long, it will lead to some disadvantages such as symptoms of lassitude, fatigue, and decreased metabolic rate, same as those with lacking sleeping or insomnia.

On a one-day cycle, the night is the time of hiding and storing for yang qi. The yang qi of the human body needs to be accumulated and supplemented through sleep. If one person is disturbed while sleeping soundly in the deep night, he or she will feel extremely uncomfortable, and the next day he or she will be listless. This is the outcome of yang qi failing to store and hide in human body. If for a long time, he or she will completely breaks down.