How TCM describe sleep? Like I said in How Traditional Chinese Medicine (TCM) look at Menstruation : Traditional Chinese Medicine uses the natural world as a metaphor for the human body. Qi (the energy) is like the water, when the sun (active energy) rise, it gets heated and evaporate to become cloud that flows in the sky. When the sun sets, it become cold, and fall down to the earth like the rain or the fog. It seeps into the earth, and hydrate everything that's growing in it. So the distribution of Qi controls the biological clock in our body: When we wake up from the sleep, the Qi must be activated and start to spread out under our skin, and supply our eyes, ears and nose. If it isn't activated enough, it'll be like a raining day: we'll feel tired and cold. When we want to fall asleep, the Qi needs to be deactivated, start to seep into our body and hydrate everything within. If it isn't deactivated enough, it'll be like perpetual day: we won't be able to sleep. If there's any reason cause the Qi hard to seep into our body, it well become a light sleep, and make us wake up tired.