Circadian Rhythm

Circadian Rhythm

The circadian rhythm describes the internal biological clock that controls an organism's daily activity cycles. It is well-known that in many species, activity does in fact follow daily patterns. This applies to activities such as feeding or sleeping, as well as to physiological attributes such as metabolic rate, body temperature, blood pressure, and hormone levels. Most animal and plant species have daily rhythms and even fairly simple creatures like bacteria have been known to have natural daily cycles.

Experiments in which organisms are placed in constantly light or constantly dark environments frequently indicate that daily periodicity in activity patterns persists even when the normal day-night cues from sunlight are removed. These circadian rhythms are important in allowing for the timing of important daily activities.

Circadian Cycles

Circadian rhythms typically describe a twenty-four-hour cycle that corresponds to the length of the day on Earth. However, for almost all species, the "natural" cycle is either a little shorter or a little longer than twenty-four hours. This explains the origin of the term "circadian" rhythm: circa means "about" and diem refers to "day." Thus, circadian means "about a day."

Changing light cues from the external environment allow organisms to adjust their natural body clocks and conform to a twenty-four-hour day. Research on circadian rhythms in humans has shown that the human body naturally drifts toward a twenty-five-hour cycle in constant-light conditions.

The ways in which circadian rhythms are controlled vary among biological organisms. In many vertebrate species, circadian rhythms are controlled by the hormone melatonin. Melatonin production follows a daily cycle, high during the night hours and low during daylight hours. (Because increased levels of melatonin production cause sleepiness, it is sometimes used as a sleeping aid.) Melatonin is produced by endocrine cells in the pineal gland, which is located in the central part of the brain. Ultimately, melatonin cycling responds to light cues from the environment.

In some species, such as certain fishes and lizards, a minute hole in the otherwise bony skull allows light to pass directly to the pineal gland. In this way, the pineal gland receives direct information from the external environment on day-night cycles. However, direct exposure of light to the pineal gland is not necessary. In birds, photoreceptors in the eyes as well as in the brain are responsible for transmitting signals to the part of the brain that controls circadian rhythms.

In mammals, the eyes alone appear to be responsible for photoreception related to circadian cycles. This involves a special pigment (a light-absorbing molecule) called crytochrome that is present in mammalian eyes. Crytochrome is distinct from the pigments that are responsible for vision, and is also found in a different part of the retina. In certain cases of blindness, the circadian rhythm is not disrupted because the parts of the retina that contain crytochrome are intact.

Problems Linked to Circadian Rhythm

Circadian rhythms impact everyday human life in many ways.

Jet lag.

Jet lag, which can be the result of travel across several time zones, is caused by discrepancies between an individual's internal clock and signals provided by the external environment. The body generally adjusts to jet lag over the course of a few days as it is exposed to daylight patterns in the new time zone. Working the night shift can also be problematic for humans, because humans are naturally diurnal , that is, active during the day and asleep at night. Data indicate that the majority of industrial accidents occur at night, when the body is programmed to slow down in preparation for sleep.

Seasonal affective disorder (SAD).

Seasonal affective disorder (SAD), which can result in extreme depression during the winter months, may also be related to the operation of circadian clocks. SAD strikes when the days are short and light is scarce. SAD is particularly common at high latitudes, where the days are particularly short during the winter. Short light cycles are likely to disrupt the circadian clock, although the link to depression has not been demonstrated. Regular exposure to artificial bright lights is often effective in treating SAD.

Sleep disorders.

Certain sleep disorders, not surprisingly, are related to problems with circadian rhythms. Older people, for example, produce less melatonin. This may be linked to the sleeping patterns associated with age, such as insomnia, early rising, and sleepiness during the day. Delayed sleep phase syndrome (DSPS), which has also been linked to old age, is a more serious problem that causes people to want to sleep from early morning to noon, instead of during more typical night hours.

Circadian rhythms are only one example of the broader category of biological rhythms. Biological rhythms range widely in duration, with circadian rhythms being among the shortest. Other biological activities follow longer cycles, such as lunar cycles, or even periods of several months or a year. The menstrual cycle and seasonal mating activity are examples of processes that occur cyclicly, but over a longer time period.

see also Diurnal; Nocturnal.

Jennifer Yeh

Bibliography

Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton, 1996.

Withers, Philip C. Comparative Animal Physiology. Fort Worth, TX: Saunders College Publishing, 1992.