Circadian Rhythms Research Paper

1. What Are Circadian Rhythms?

All our behavioral, physiological, and endocrinological functions are controlled by an endogenous clock that measures time in approximately 24-hour intervals. The rhythms the clock generates are known as circadian rhythms (from the Latin, circa, about, and dies, day). The ‘about’ is very important. Living systems do not merely respond to cyclic changes in their environment in some phase-locked fashion. Leaves of plants that open during the day and close at night still open and close when the plants are kept in constant darkness. Humans and other animals maintain a period of about 24 hours in bodily functions when they are in environments without time cues. Noise, temperature, and social stimuli can also synchronize, or entrain, the clock, but the strongest entraining signal is the light dark (LD) cycle.

2. What Are Their Properties?

Figure 1 illustrates two important properties of circadian rhythms in the wheel-running activity of a nocturnal mammal, the white-footed mouse (Peromyscus leucopus), that has been placed in a 24-hour LD photoperiod consisting of one hour of light and 23 hours of darkness. It takes the mouse about a week for its activity to become entrained by the LD cycle, after which it begins running immediately after the lights go off. It runs for about 10 hours and then becomes inactive. After 60 days the animal is released into constant darkness. There its activity free-runs with a period of 23.6 hours, that is, it begins to run 24 minutes earlier each day.

After two weeks in the dark, the mouse is put into a 24-hour LD cycle consisting of 18 hours of light and six hours of darkness. It immediately becomes entrained to the new photoperiod and its activity is tightly confined to the dark period. When it is released back into constant dark, two aspects of its activity are different from the previous stay in the same condition. First, the amount of its activity is lessened. Second, the period of its free-running rhythm is shorter—23 instead of 23.6 hours (Pittendrigh 1974). This indicates that the clock remembers the previous LD cycle and this influences both the amplitude and period of the free-running rhythm. A ‘memory’ of the preceding daylength is even encoded in the electrophysiological activity of an isolated slice of brain containing the SCN (Mrugala et al. 2000).

Rhythms have different phase relations to the LD cycle. Figure 2 shows entrained rhythms in several physiological parameters in a human subject in a 16:8 LD cycle. Body temperature is high during the day and falls at night, when the subject is asleep. Cortisol is highest at the end of the dark period. Growth hormone is low during the day and shows a sharp peak at the beginning of the dark period, during sleep. Immune system components also have circadian frequencies, and this can be very important in determining both responses to antigens and the timing of treatments such as chemotherapy in cancer. Ideally, one would want to administer a therapeutic agent at a time when it would produce the fewest unwanted side effects and yet be most effective in attacking the dividing cell.

3. Why Do We Need Circadian Rhythms?

There is a distinct advantage in timing various behaviors and metabolic processes to the appropriate time of day. Organisms have to sleep, and they generally do so either in the day or at night. One would want potential mates and food to be available when one is active. But a simple hourglass mechanism, timing precise 24-hour periods, would be inadequate because of seasonal changes in day length. A circadian clock, sensitive to external conditions, can be reset each day, making the clock conform to the environment. Furthermore, an internal clock allows organisms not only to respond to changes in the environment but also to anticipate them. Lizards gain heat by basking in the sun. When they are in their burrows, still with low body temperatures, they crawl to the opening of the burrow and stick their heads above the desert floor before the sun comes up. This way they are in a position to gain enough heat to become active as soon as possible. The early bird does catch the worm, and its internal clock wakes it up before dawn. The body temperature of humans falls in the evening, while we are still awake, and begins to rise in the early morning, while we are still asleep. Even in the same entrained environment, people have different phase relations with the LD cycle; some are larks and some are owls.

4. What Controls Circadian Rhythms?

Almost all known plants and animals exhibit circadian rhythms. For instance, the microscopic single-celled aquatic plant, Gonyaulax polyedra, is phosphorescent, lighting up at night, and dimming in the day. In 1958, Hastings and Sweeney demonstrated that Gonyaulax showed peaks and troughs of luminescence even in constant darkness, but the peak time of luminescence shifted a little later each day. If the plant was exposed to brief pulses of light, the peak of luminescence could be shifted to almost any time of day, depending on when the light pulse was given. Thus, light reset the Gonyaulax clock.

In 1972, Moore and Lenn placed a radioactive label into the eyes of rats and found that there was a direct pathway from the retina to two tiny nuclei in the hypothalamus. These nuclei lie behind the eyes right above the location in the brain where fibers from the left and right retinas cross, the optic chiasm, and therefore the nuclei are named the SCN. In that same year, Stephan and Zucker showed that lesions of the SCN could permanently eliminate or weaken circadian patterns of behavior (for a review, see Rusak and Zucker 1979). In a series of papers beginning in 1987, Lehman and his colleagues demonstrated that locomotor activity rhythmicity can be restored in hamsters with SCN lesions by transplants of fetal SCN tissue. There is now no doubt that, for most rhythms, the SCN is the main clock in the mammalian brain. Each morning, light from the eye sends electrical signals to the SCN and resets it. The SCN, in turn, synchronizes the rest of the brain and sets the pace for all daily activity patterns, just as a conductor synchronizes all the instruments in an orchestra.

The analogy of the SCN to an orchestra conductor is inadequate, however, because the cells of the SCN do not act as a single multioscillator unit. Welsh et al. (1995) demonstrated that SCN cells grown in culture oscillate at different rates: this means that each single SCN neuron functions as an independent circadian clock.

5. Are Clocks Similar Throughout The Animal Kingdom?

Clocks are being dissected rapidly through genetic analysis. Clock genes have been identified in cyanobacteria (blue–green algae), the fungus Neurospora, and the fruit fly Drosophila. In 1988, Ralph and Menaker (1988) identified a tau mutation in Syrian hamsters. Hamsters homozygous for the mutation had free-running activity rhythms of about 20 hours, compared to the wild type hamster’s rhythm of about 24 hours. In 1994, Takahashi’s laboratory discovered the first mouse circadian mutant, Clock, which was arrhythmic (Vitaterna et al. 1994). The Drosophila clock is proving highly amenable to molecular analysis and there are close connections to the mouse mutation. Many clock genes are highly conserved: some of the same molecules are present in fruit flies and mice, and it is possible that common molecular clock mechanisms from bacteria to humans will be found in the not-too-distant future (see Young 2000, for a review).

6. How Do Circadian Rhythms Develop And Age?

Using 14C-labeled deoxyglucose to monitor metabolic activity in the SCN, Reppert and Schwartz (1986) found that there was a distinct day–night oscillation of metabolic activity in both rat and primate SCN. But fetuses do not process light. They found in rats that the maternal circadian system coordinated the timing of the fetal rhythms. It takes about a month after birth for strong circadian rhythms to develop. In most, but not all individuals, the robustness of the rhythms declines with age. Thus, rats that normally sleep mostly in the light, and eat and drink mainly in the dark, distribute all these activities about equally in the light and the dark when they get old. The mean amount of sleep, food eaten, and water drunk, etc., stays the same: only the pattern changes. These changes are correlated with aberrant SCN firing patterns in SCN slices taken from old rats. This implies that aging could either disrupt coupling between SCN pacemaker cells or their output, or cause deterioration of the pacemaking properties of the individual cells (Satinoff et al. 1993). A better understanding of clock mechanisms should lead to more efficacious clinical treatments for alleviating disorders in the elderly, such as sleep disturbances, that are associated with changes in the clock itself. But aside from pathologies in aging, a more complete understanding of clocks and how they are entrained by light will be useful in helping to ameliorate the discomfort and disability that comes from conditions as disparate as jet lag, sleep disorders in shift workers and blind people, and depression and manic-depressive disorder.

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