Biological rhythm, observed in vital activities in humans and all living creatures, is a function which allow us to adapt ourselves to environment and basically synchronized with natural rhythms.

Traditional Chinese medicine (TCM) explores application of biological rhythm and human rhythmic changes since Internal Medicine of Yellow Emperor – Huangdi Neijing 黄帝内经 (475-221 BC). It includes circadian rhythm - physical, mental, and behavioral changes that follow a daily cycle (sleeping at night and being awake during the day is an example of a light-related circadian rhythm) a syzygial rhythm - lunar rhythm and seasonal rhythm - annual rhythm, exploring harmony between humans and nature.

Harmony between humans and nature depends from structures and composition compatibility. It was stated in Lingshu Jing 灵枢经 - Spiritual Pivot (ca. 46 BCE-AD 23), that in human body, five Zang (yin) organs are compatible with five musical scales of heaven, six Fu (yang) organs are compatible with six tunes of heaven, 365 acu-points are compatible with 365 days a year; knees and shoulders are compatible with mountains on the land, etc.

Humans and the nature are based on biological rhythms in TCM, and according to situations they share the very same characteristics and compatibility, e.g. five zang (solid) organs can produce happiness, anger, thoughts, sorrow and fear, while nature produces coldness, heat, dryness, wetness and wind.

Humans and nature exchange information and energy, and mutually influence each other. For more than 2000 years TCM considers that climatic changes influences humans, e.g. sky (air) communicates to lungs, earth to throat, wind to liver, thunder to heart, grain to spleen and rain to kidney. For instance: patient with liver diseases feels fresh in the morning, aggravated in the evening and relieved at midnight.

TCM believes that nutrients - Ying Qi, circulates within pulse and defensive materials - Wei Qi circulates out of pulse continuously. Nutrients circulates fifty cycles along the body within one day and one night, and defensive materials circulate along skin during daytime for 25 circles and the internal body during night for 25 circles. The circulation of Defensive Qi dominates human sleeping/awakening cycle.

TCM four seasons theory is easily applied on daily rhythm. One day is divided into four seasons, starting with morning as the spring, noon as the summer, sunset as the fall and midnight as the winter. Following this pattern, TCM considers that in humans qi emerges in the morning, grows at the noon, declines in the evening and enters organs at midnight.

Full moon describes time when accumulation of blood qi provokes circulation of Defensive Qi, and strengthens blood nutrients and muscles. Consequently, at the wane, muscles and meridians are weakened, when Defensive Qi is mostly depleted.

During full moon sea water tides to the west and it does influence people in a way that cause accumulation and strength of qi and blood, better circulation in muscles, fresher hair and skin, in short - produces vitality. At the wane, sea water tides move to the east, making weak qi and blood, muscles without elasticity, flabby skin, dying hair, in general, state without vitality. Old Chinese found that moon phases influence human body, spirit and minds, such as blood and qi (energy) activation or declination.

Humans biological rhythm influences our internal organs. TCM believes that liver function is greatly promoted in spring, heart in summer, lung in autumn and kidney in winter.

Seasonal changes influence internal organs following particular pattern. First and 2nd lunar months influence liver, 3rd and 4th lunar months influence spleen, 5th and 6th influence head. Seventh and 8th lunar months influence lungs, 9th and 10th influence heart, and kidneys at lunar months 11th and 12th. Chinese Medicine says that twelve meridians correspond to the 12 months, and 10-day rhythm is basic time unit used for prognosis and evaluation of diseases.

According to ancient Chinese theory, 24 hours period consist of day (yang – active phase) and night (yin – passive phase), and the day and night could also divided into two periods, yang and yin. The period from early morning to noon is “yang (rise) of yang (day)”, from noon to evening is “yin of yang”, from evening to crow is “yin of yin” and from crow to early morning is “yang of yin”. Four periods during one day and one night have their yin and yang features respectively.

So, lets mention some practical examples of natural biorhythm. Heart as the example, has its most active phase from early morning to noon, and researches prove that blood pressure and heart rate reach its peak until noon, as the incidence of heart diseases and sudden cardiac death.

From the point of view in TCM, the whole body is in a state of “yang of yang” or the highest state of yang when cardiac burden rises to the highest point, and risks of exhausted heart can lead to highest incidence of morbidity.

Lung diseases mostly occur in the afternoon with risk of complication and death, kidney diseases from evening to midnight, and liver diseases from midnight to dawn.

Clinical researches proved that TCM theoretical foundation is correct, in particular related to biological circadian rhythm. For instance, severe liver disease has the highest mortality during the period from 15 to 19 o'clock.

Traditional Chinese Medicine in order to explain seasonal rhythm uses Five Element Theory to understand seasonal rhythm. Each season has its Five-elements features of its own, such as spring corresponding to wood, summer to fire, long summer to soil, autumn to gold, and winter to water. Similarly, Five solid organs also have their five-element features respectively, such as liver belonging to wood, heart to fire, spleen to earth, lung to metal and kidney to water.

Seasonal rhythm of rotating duty states that liver dominates spring, heart summer, spleen long summer, lung autumn and kidney winter. Therefore, we can use Five Element Theory to investigate and understand seasonal rhythm in people.

Seasonal rhythms of diseases can be very visible from the point of biological seasonal rhythms. For example, spring is the season of high incidence of depression. This can be explained based on TCM syndrome differentiation, when liver is obstructed and has deficiency feature found in most depressive patients. According to TCM spring belongs to wood, promoting growth and germination. Our body should enter “wood” state relying on growth function of liver. Therefore, in the case of liver deficiency, body requirement can not be satisfied and exterior requirement may in turn aggravate exhaustion of liver qi, resulting in depression state with the failure of body's entry into “genesis and growth” rhythm.

Biological rhythm obstruction can be a great source of diseases. TCM believes that pathology is produced by internal factors (yin) or external factors (yang). External factors are mostly climatic ones, such as: wind, rain, cold, humidity, and heat, while internal factors are: food, domicile, and extreme emotion like happiness, depression, sorrow, over-joy and anger. Improper food intake at regular time, and non-regular daily lives are some reasons for production of disease evils, when the pathogen mechanism cause our living habits to destroy normal rhythms causing imbalance between yin and yang, pathology in Five Elements, and finally, physically expressed disease.

It is clinically proved that destruction of biological rhythm may induce or accelerate diseases caused by light pollution during night, or shift work may induce disorder in metabolism and therefore cause multiple diseases such as obesity, diabetes or metabolic syndromes.

Scientific medicine gives priority to physical pathological changes, having aim to give structural explanation of diseases. On the other hand, TCM applies comprehensive point of view to diseases time development, space interactions and influences on humans, understanding biological rhythms in a whole point of view