How is chemistry involved in sleeping?

The human body is a large, complex biochemical machine. The chemistry of life is associated with us at all times between birth and death. We do not completely understand all the chemical processes in the body, and the chemistry of the brain is still being investigated as well. Brain chemistry changes as a result of different external factors, and the brain itself can change its own chemistry. All of this is still the subject of investigation because of the complexity of the body and the brain, and the holes in our knowledge of the chemistry of those biological systems.