Did you know that your body has its own internal network of clocks? These biological clocks help you feel alert during the day, hungry at mealtimes, and sleepy at night. Keeping your body's daily cycles, or circadian rhythms, in sync is important for your health. "Circadian rhythms are big influencers in the body," explains Dr. Michael Sesma, an expert in circadian biology. "They affect almost every part of your physiology in one way or another. Learning how the rhythm is generated is critical for understanding health."
Our natural daily rhythms are synchronized with the sun. A "master clock" in the brain receives direct input from the eyes and coordinates all the biological clocks in the body. During the day, it sends signals to other brain regions to make hormones that will help keep you awake, boost your heart rate, and give you energy. In the evening, when less light enters your eyes, it triggers production of a hormone called melatonin. Melatonin makes you feel drowsy and helps you stay asleep.
"Many of your body's functions and normal daily activities—like sleeping, waking, and eating—are patterned around this 24-hour cycle," Sesma explains.
Your biological clock's "settings" are determined by specific genes. These settings can affect body temperature, blood pressure, activity level, inflammation (your body's protective response to injury or infection), mood, and brain functions.
Be mindful about how you may be altering your circadian rhythms. "Our around-the-clock society creates challenges for our internal clocks," Sesma says. "There are lots of modern situations that can disrupt our rhythms, and some may contribute to health problems. Researchers are considering time of day and how to sync up with the body's clocks in all aspects of health." These studies may lead to new insights for a range of clock-related disorders, from insomnia to jet lag.
Which citation identifies the main source of information for this passage?

A. The National Institutes of Health. "Understanding Your Daily Rhythms." NIH: News in Health. 11 April 2018. https://newsinhealth. nih. gov/2018/04/tick-tock-your-body-clo cks. Accessed 11 February 2019.

B. Master Clock. U. S. Department of Health and Human Services, 2018.

C. "Circadian Rhythm." Encyclopedia Brittanica. https://www. britannica. com/science/circadian-rhythm. Accessed 11 February 2019.

D. Sesma, Michael. Personal interview. 11 April 2018.