Circadian Rhythms
Circadian rhythms are regular changes in mental and physical characteristics that occur in the course of a day (circadian is Latin for “around a day”). Most circadian rhythms are controlled by the body’s biological “clock,” known as the suprachiasmatic nucleus (SCN), which is located roughly at the center of the base of the brain. THE SCN receives input from the eye, which transmits information about light levels. In this way, the SCN can stay synchronized to the 24 hour day-night cycle. One important function of the SCN is to deliver an alerting signal that rises and falls across the 24-hour day.
Signals from the SCN travel to several brain regions, including the pineal gland, which responds to light-induced signals by switching off production of the hormone melatonin. The body’s level of melatonin normally increases as darkness falls, making people feel drowsy. The SCN also governs functions that are in line with the sleep/wake cycle, including body temperature, hormone secretion and changes in blood pressure.
When travelers pass from one time zone to another, they suffer from disrupted circadian rhythms, an uncomfortable feeling known as jet lag. For instance, if you travel from California to New York, you "lose" three hours according to your body’s clock. You will feel tired when the alarm rings at 8 a.m. the next morning because, according to your body’s clock, it is still 5 a.m. It usually takes several days for your body's cycles to adjust to the new time.
To reduce the effects of jet lag, some doctors try to change the biological clock with a technique called light therapy. They expose people to special lights, many times brighter than ordinary household light, near the beginning or end of the day, depending on whether the individual needs to shift to an earlier wake-up time or later bedtime, respectively. This helps them reset their biological clocks and adjust to a new time zone.
Symptoms much like jet lag are common in people who work nights or who perform shift work. Because these people’s work schedules are at odds with powerful sleep-regulating cues like sunlight, they often become uncontrollably drowsy during work at night, and they may suffer insomnia or other problems when they try to sleep during the day. Shift workers have an increased risk of heart, digestive, emotional and mental problems, all of which may be related to their sleeping problems. The number and severity of workplace accidents also tend to increase during the night shift. It may be possible to reduce shift-related fatigue by using bright lights in the workplace, minimizing shift changes, taking scheduled naps, and using medications to help regulate sleep and wakefulness.
