Circadian rhythms are changes in mental and physical characteristics that are regular and occur in the course of a day. The word circadian is from the Latin circa diem and it means about a day. However, circadian rhythms may not be daily, they may take place in multiple periods of a day or period which last longer than a day (circannual rhythms). Most circadian rhythms are controlled by a circadian pacemaker or the body's biological clock or the suprachiasmatic nucleus (SCN). Physiologic parameters are influenced by the body's circadian clock. The most commonly noted circadian rhythm is sleep. Other examples of circadian rhythms, governed by the SCN and synchronized with the sleep/wake cycle, include body temperature, blood pressure, urine production, digestive secretions, and hormone production. These rhythms are controlled by internal and environmental factors. Light and temperature are the most common environmental factors (National Institute of Neurological Disorders and Stroke [NINDS], 2003).
Signals from the SCN travel to several brain regions, including the pineal gland, which responds to light-induced signals. The body's level of melatonin increases after darkness falls, causing drowsiness. Studies have shown that since sunlight or other bright lights reset the SCN, the biological cycles normally follow the 24-hour cycle of the sun, rather than the innate cycle. Circadian rhythms can be affected by external time cues, such as the beeping of an alarm clock, the clatter of a garbage truck, or the timing of meals. When travelers pass through time zones their circadian rhythms are disrupted (jet lag). Symptoms such as jet lag are also found in those who work nights or who perform shift work. People with total blindness may experience life-long sleeping problems since their retinas are unable to detect light (NINDS, 2003).
Irregular sleep-wake patterns due to irregular circadian rhythm include the following: T...