Shen can be translated as "Spirit" or "Mind", and implies our consciousness, mental functions, mental health, vitality, and our "presence".
Shen lives in the Heart, where it retires to sleep during the night. If the Shen is disturbed, there may be insomnia. Shen is specifically said to live in the Blood Vessels (part of the system of the Heart) and to be nourished by the Blood. In TCM pathology, therefore, deficient Blood may fail to nourish the Shen. Alternatively, Heat (of various Organs) may disturb the Shen.
State of the Shen is said to be visible in the eyes. Healthy Shen produces bright, shining eyes, with vitality. Disturbed Shen produces dull eyes, which seem to have a curtain in front of them - as if no one were behind them. Often seen in those with long-term emotional problems or after serious shock (even a shock that occurred a long time ago.)
Healthy Shen depends on the strength of the Essence (stored in Kidneys) and Qi (produced by Spleen and Stomach). Thus, Shen is dependent on the Prenatal Jing and the Postnatal Jing. If Essence and Qi are healthy, the Shen will be nourished. As mentioned above, the Shen lives in the Blood Vessels, part of the Heart system in TCM. Blood is closely related to Qi in TCM, and is formed from the Postnatal Jing derived from food and fluids, hence Blood formation is simultaneous with that of the formation of Qi.
Jing, Qi and Shen are the "three treasures" in TCM. They represent three different states of condensation of Qi, ranging from Jing (more fluid, more material) to Qi, more rarefied, and Shen, more rarefied and immaterial.