In traditional Eastern thought, the origin of the universe is described as Chaos: an undifferentiated state in which heaven, earth, and all things have not yet been separated. From this primordial energy, two complementary forces emerge. The bright, light, and clear force that rises upward is called Yang, while the dark, heavy, and dense force that descends is called Yin. Although they appear opposite, Yin and Yang share the same source.

Yin is associated with receptive qualities, while Yang represents active qualities. The moon, night, and winter are traditionally classified as Yin, whereas the sun, daytime, and summer are classified as Yang. What is important is that Yin and Yang are not merely opposing concepts. Because they arise from the same source, they influence and interact with one another. It is through the balance of these two forces that order is maintained.

Traditional Eastern thought also teaches that nothing is completely Yin or completely Yang. Yang exists within Yin, and Yin exists within Yang. Under changing conditions, Yin may transform into Yang, and Yang may transform into Yin. Through this continual process of change, an overall balance is preserved.

When this perspective is applied to sleep, sleep can be viewed as Yin and wakefulness as Yang. These states are clearly different, yet they occur within the same person and constantly influence each other. A restful night's sleep promotes alertness and activity during the day, while an active and fulfilling day supports better sleep the following night. Conversely, poor sleep reduces daytime alertness, which can lead to another unsatisfactory night of sleep. Sleep and wakefulness therefore form a continuous cycle.

Yin and Yang can also be found within sleep itself. During sleep, it is primarily the brain that rests, while the brainstem remains active and helps the brain recover. Moreover, the brain does not remain in a single state throughout the night. Deep, restorative non-REM sleep alternates with the more active REM sleep. Here again, we can recognize a Yin-Yang relationship.

There is a larger Yin-Yang relationship between sleep and wakefulness, and a smaller Yin-Yang relationship within sleep itself. Maintaining balance among these complementary processes contributes to a healthy and fulfilling life. Being active during the day and resting well at night creates the natural rhythm that supports Yin-Yang balance.