Circadian Rhythms
Most biological and psychological processes follow natural rhythms. Those that have a cycle of about one day are called circadian rhythms.

What Are Circadian Rhythms?
Circadian rhythms influence our body temperature, sleep and wakefulness, and various hormonal changes. Sunlight and other time cues help to set our circadian cycles so they are consistent from day to day. For most people, the length of a complete cycle is very close to 24 hours.

Circadian rhythms are coordinated by small nuclei (centers) in the middle of the brain, the suprachiasmatic nuclei (SCN). The SCN are connected to other parts of the brain and helps control the body’s temperature, hormone release, and many other functions. A pathway runs from our eyes to the SCN, and light seems to play the largest role in setting our circadian clocks. Interestingly, blind people often report problems with circadian rhythms, since it is difficult for them to get the time cues needed to set their circadian clocks. Other factors they may affect the SCN and the setting of the circadian clock include exercise, hormones, and medications.
In healthy people, the various circadian rhythms are “in tune” like the many instruments of an orchestra. Body temperature, for example, starts to rise during the last hours of sleep, just before waking up. This may promote a feeling of alertness in the morning. In the evening, body temperature decreases in preparation for sleep. A small drop in temperature also occurs in most people between 2:00 to 4:00 p.m., which may explain why many people feel sleepy in the early afternoon. Although it has not been proven that changes in body temperature determine our sleep habits, there does appear to be a relationship between the two.

What Causes Circadian Rhythm Disorders?
To a large extent, an individuals, an individual’s circadian system seems to be determined by genetics. Age-related changes in the circadian system also appear to affect the natural rhythm and ability to respond to time cues. These factors can lead to a conflict between the body’s sleep signals and the demands of society. We are just beginning to understand how the circadian system functions, and to address problems and treatments related to the circadian system. Sleep laboratory results show that sleep generally consists of a normal progression of stages and tends to occur in a single nighttime block. Unfortunately, work, school, and social commitments may not coincide with a person’s natural circadian cycle. Or, if a person’s circadian rhythms changed significantly, it may become difficult to cope with society’s regular demands. Numerous circumstances or factors can cause the “circadian orchestra” to fall out of sync.

  • Jet Lag: The most widely experienced circadian problem is jet lag, which occurs when a person travels across several time zones. A typical flight from the United States to Europe, for example, often produces jet lag symptoms that can last for a week or longer. These include insomnia, daytime sleepiness, indigestion, irritability, and poor concentration. Some people require up to a week to adjust to new time cues; some adapt more quickly, depending on the number of time zones involved. Most of us experience a mild form of “jet lag” twice a year during the switch to and from daylight-savings time.

  • Shift work: Shift workers are employees who work nontraditional hours, such as night shift or rotating shifts. These workers often face problems similar to jet lag without ever leaving home. People who work the night shift have to adjust to an unnatural schedule of working while others are sleeping and sleeping while others are awake. In addition, they may not get as much sleep during the day as daytime workers get at night. Their sleep is often fragmented during the daytime because the brain is active and programmed to be awake. People who work rotating shifts often find it difficult to get enough sleep, since their work schedules change frequently.

  • Delayed sleep phase syndrome (DSPS): Some people find that they are not able to fall asleep until 2:00 am or later, and then have trouble waking up in time for work or school. Few lifestyles allow this kind of sleep/wake schedule. This problem which is more common in young adults than in other age groups, can interfere with employment and school, and can lead to psychological stress.
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