The four factors that can be defined as essential parts of one’s overall health are social, mental and emotional, and physical. The answer to your question is C. I hope this is the answer that you are looking for and it comes to your help.
Answer: C. Melatonin.
Explanation: Melatonin is the hormone responsible for sleep and wake cycles. :)
You didn't really provide us with any options? It must be noted that generally, the structure of the skeletal system also dictates its function. With that I mean that having bones and a skeletal system enables us to move more efficiently and to have the pose which we have.