• 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 constant 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) at the base of the brain, the suprachiasmatic nuclei (SCN). The SCN is 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 that 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 seems to promote a feeling of alertness in the morning. In the evening, body temperature decreases in preparation for sleep. A drop in temperature also occurs in most people between 2:00 and 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 our 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 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 eight-hour 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. The next section lists some that may be responsible.

    Circadian Rhythm Disorders

    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-saving time.

    Shift Work: Shift workers are employees who work nontraditional hours, such as night shifts 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 or 3:00 am. and then have trouble waking up in time for work or school. Few lifestyles allow for 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.

    Advanced sleep phase syndrome (ASPS): This syndrome is more common among older adults, and has only recently been recognized as a significant problem. Sleepiness usually begins in the early afternoon, and sufferers often wake up too early and then aren't able to go back to sleep.

    Because ASPS usually doesn't interfere with working hours, society is more tolerant of this problem than of DSPS. ASPS becomes a problem, however, when sleepiness interferes with plans for evening business or social commitments. As in DSPS, lack of sleep does little to remedy this problem. ASPS sufferers would continue to wake up early even if they forced themselves to stay awake until later in the evening.

    Irregular Sleep/Wake Pattern:Some people find that their sleep/wake cycles cannot adjust to a 24-hour period, however hard they try. Bedtimes may be very irregular or continue to drift later and later, resulting in a variety of problems similar to those encountered with jet lag.

    What Treatments Can Help Circadian Rhythm Disorders?

    Certain adjustments to the sleep schedule can help travelers and shift workers alter their circadian rhythms. For example, those working rotating shift should on the last few days of the evening shift, delay bedtimes and wake-up times by one to two hours. As the night shift begins, workers will already be well on their way to adapting to the new schedule. Travelers and shift workers should also try the following:

    • Try to allow extra time for adjustment during a trip or when switching to a new work schedule. Don't skimp on time for resting.
    • Depending upon the new time zone, a short nap at a specific time of day can be useful in helping overcome jet lag.
    For most other circadian rhythm disorders the following suggestions could help:
    • In some cases, an abnormal sleep cycle can be a symptom of depression or poor sleep habits. Evaluation by a professional can lead to proper treatment.
    • Bright-light therapy is being studied as a way to shift the circadian system and reset the body's clock. Properly timed exposure to bright lights may help advance or delay the sleep cycle.

    In general, evening exposure to bright light is used to treat ASPS by shifting the circadian clock to a later hour. Morning exposure to bright light is used to treat DSPS by shifting the circadian clock to an earlier hour. If you think you suffer from this disorder, seek specific advice about this from your healthcare professional.

    Some researches have explored the use of supplemental melatonin, a naturally occurring substance that increases in the bloodstream during the night. Although this form of treatment is experimental, it is believed to help promote sleep onset in some situations.

    As with any other sleep disorder, talk to your healthcare professional about your symptoms and the appropriate treatment options for you.

    Courtesy of the American Academy of Sleep Medicine (http://www.aasmnet.org)

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