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Author:

Michael B. Russo, MD

Coauthor:

Shehnaz Shaikh, MD

Editor:

William C. Shiel Jr., MD, FACP, FACR

William C. Shiel Jr., MD, FACP, FACR

Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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Biological variations that occur in the course of 24 hours are called circadian rhythms. Circadian rhythms are controlled by the body's biological clock. Many bodily functions follow the biologic clock, but sleep and wakefulness comprise the most important circadian rhythm. Circadian sleep rhythm is one of the several body rhythms modulated by the hypothalamus (a part of the brain).

Light directly affects the circadian sleep rhythm. Light is called a "zeitgeber," a German word meaning time-giver, because it sets the biological clock. A practical purpose has been proposed for the circadian rhythm, using the analogy of the brain being somewhat like a battery charging during sleep and discharging during wakefulness.

Body temperature cycles are also under control of the hypothalamus. An increase in body temperature is seen during the course of the day and a decrease is observed during the night. The temperature peaks and troughs are thought to mirror the sleep rhythm. People who are alert late in the evening (evening types) have body temperature peaks late in the evening, while those who find themselves most alert early in the morning (morning types) have body temperature peaks early in the evening.

Melatonin (a chemical produced by the pineal gland in the brain) has been implicated as a modulator of light entrainment. It is secreted maximally during the night. Prolactin, testosterone, and growth hormone also demonstrate circadian rhythms, with maximal secretion during the night.

Circadian rhythms can be affected to a certain degree by almost any kind of external stimulus, for example, the beeping of the alarm clock or the timing of meals. When we cross time zones, our circadian rhythms get disrupted leading to jet lag. It usually takes several days for our body rhythms to adjust to the new time.

Symptoms similar to those seen in people with jet lag are common in people who work during nights or work in shifts. Because these people's wake time conflicts with powerful sleep-regulating cues like sunlight, they often become uncontrollably drowsy during work or may have difficulty falling asleep during their off time. Their biological clock wants to do one thing while they are doing something entirely different. People working in shifts have an increased risk of heart, gastrointestinal, emotional, and mental problems. All these problems may be related to the disruption of the circadian sleep rhythm.

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