Overtiredness causes weak Yang chi too. Chinese medicine says that fatigue depletes Yang chi/energy. This depletion is invisible. You can’t see it or touch it, but it’ll be shown on the body. For example: people who stay up late into the night may wake up in the morning, look in the mirror, and notice their tongue has a thick coat. This shows that their body is heated. After burning the midnight oil their appetite is usually not good because the spleen is strained.