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Biological Rhythms

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  • Pages: 11
  • Word count: 2530
  • Category: Biology

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Many squirrels hibernate every year, many birds migrate south for the winter, and crickets start their mating calls each day before sunset. Many plant activities, like transpiration and synthesis of certain enzymes, happen within the course of a day. But what drives these cycles of behavior is the organisms’ internal clocks that help govern behavior on time scales that run from a few minutes to a year.

In order to say that such a clock exists, it is essential to show that periodic behaviors really are internal – not triggered by external cues such as the changes in light levels, temperature, and relative humidity. To prove that these conditions are not effective on internal clocks, biologists have done their researches on lab conditions, where the animals can be isolated from any environmental cue. Even in this artificially constant conditions, many physiological processes continue to oscillate with their periodicities. For instance, many legumes lower their leaves in the evening and raise them in the morning. A bean plant will continue these “sleep movements” even if kept in constant light or constant darkness; the leaves are not simply responding to sunrise and sunset.1 Another example is the male teleogryllus cricket, who continue to chirp nearly 11 hours per day and start every bout of calling approximately 25 or 26 hours after the end of the previous bout, in lab conditions where temperature is held constant and light kept on about a clock. The point here that this biorhythmic behaviors are prompted by an internal clock and will continue in a fairly fixed way in the absence of any environmental cause.

Such physiological cycles with a frequency of about 24 hours and are not directly paced by any known environmental variable are called circadian rhythms. (From the Latin circa, approximately, and dies, day)

Figure 1: Circadian rhythm, a graphic depiction of cortisol values over a 24-hour period.

From E.D. Weitzman et al., “Twenty-four-hour Patterns of the Episodic Secretion of Cortisol in Normal Subjects,” Journal of Clinical Endocrinology and Metabolism, vol. 33, pp. 13-22, (c) by The Endocrine Society, 1971

Figure 2:

Five-day segments of simultaneous records of rectal temperature, plasma urea concentration, and plasma cholesterol concentration of a female goat (Capra hircus). The horizontal bars at the top indicate the timing of the light-dark cycle. Notice that the rhythms of rectal temperature and urea concentration have similar phases (peaking in the middle of the night) but the rhythm of cholesterol concentration has the opposite phase (peaking in the middle of the day).

But can we say that circadian rhythms are only a consequence of internal clock, can’t there be any effect of environment on the organism? Organisms, including plants and humans, continue their rhythms when placed in the deepest mine shafts or when orbited in satellites but all research thus far indicates that the oscillator for circadian rhythms is endogenous (internal) . This internal clock, however, is entrained (set)to a period of precisely 24 hours by daily signals from the environment.3 When an organism is kept under constant conditions, its circadian rhythm deviates from a perfect 24-hour. Note that the cricket’s internal clock does not keep a strict 24-hour time, but takes approximately 25 or 26 hours to complete a full cycle. This time may vary from 21 hours to 27 depending on the nature of the response. In humans, researchers suggest that the internal clock operates on a 25-hour cycle, which means we would have a 25-hour sleep-wake cycle if we were isolated in a dark room. This frequency is considerably variable, some individual have 28-hour sleep-wake cycles whereas some others have less than 24-hours.

This deviation doesn’t indicate that these free-running periods are imperfect. Free-running periods are still keeping perfect time but they are not synchronized with the outside world.

5Figure 3: Some of the key properties of a circadian oscillator

Besides these, just how our body controls its internal clocks are still mysterious. Researches suggest that that the brain, particularly one region, suprachiasmatic nucleus, has the major role in coordinating several core rhythms. The suprachiasmatic nucleus is tissue of nerve cells in the region called hypothalamus. It may regulate other centers of control, therefore is often referred to as the “master clock”. Ultimate control of the SCN is thought to reside in a gland in the brain known as the pineal gland. 6The pineal gland secretes melatonin. As soon as our brains receive sensory signals from our eyes about the waning light at sunset, for instance, the pineal gland steps up its rate of melatonin secretion. Molecules of melatonin are transported to certain brain neurons which are involved in sleep behavior, a lowering of body temperature, and possibly other physiological events. When sun rises, as the eye detects the light of a new day, melatonin production slows down. The temperature of our bodies increases, so we wake up and become active.

Figure 4:


8Figure 5 (Next Page):

Photomicrograph of the master circadian clock, the suprachiasmatic nucleus (SCN), and examples of overt circadian rhythms in several mammalian species. Upper left panel: coronal section of Nissl stained fetal sheep bilateral suprachiasmatic nuclei (SCN; one is indicated by white arrows); V: third ventricle, bar: 500 m. Upper right panel: melatonin rhythm in pregnant ewes and their fetuses (closed and open circles, respectively); (mean S.E.; n=4). Lower left panel: double plot of locomotor activity rhythm in a rat. Each line represents the recordings of two successive days. Initial recordings were done under ad libitum feeding in which the rat shows a nocturnal pattern of activity. The arrow indicates initiation of a restricted pattern of feeding in which food was available from 10:00 to 11:00 hours. Note the shift in the activity rhythm. Lower right panel: body temperature rhythm in adult capuchin monkeys (mean S.E.; n=4). Dark bars indicate hours without light.

Circadian rhythms are essential for eukaryotic life. For instance the pulse, blood pressure, temperature, rate of cell division, blood cell count, alertness, urine composition, metabolic rate, sex drive, and responsiveness to medications all fluctuate in circadian manner.9Therefore are one of the most important issues of human body.

Some of the physiological processes that fluctuate within a biological rhythm can be exampled as listed below:

* Body temperature varies during a 24-hour period by as much as 0.5?C .

Figure 6

* Blood pressure may change as much as 20% during the day.

Figure 7

* The number of white blood cells can vary by 50% during the day.

* Many hormones follow rhythmic cycles.

o The male sex hormone testosterone follows a 24-hour cycle. The highest levels occur in the night, particularly during dream sleep also known as REM sleep.

o The menstrual cycle, for instance, is a recurring series of events in the reproductive functions of women that lasts, on average, 28 days. During the menstrual cycle, levels of female sex hormone estrogen undergo dramatic shifts. Estrogen concentrations in the blood are low at the beginning of each cycle and peak on day 14, when ovulation normally occurs. Throughout the remaining 14 days, estrogen levelsare rather high. Then they drop off when a new cycle begins. Estogen levels follow this cycle month after month in women of reproductive age.

* The skin reacts differently depending on the time of the day13

o Morning:

Fewer reactions to allergens

Greater reactions to tuberculin tests

Higher skin penetration of nicotin

o Afternoon:

Higher rate of lidocaine uptake

Analgesic effect 2-3 times longer

Better effects of corticosteroid creams

o Late evening and night:

Histamine sensitivity

Itching (atopis dermatitis)

Epidermal cell proliferation (psoriasis)

Cancer risk after contact with carcinogens

14Figure 8:

A schedule of daily rhythms

This schedule reflects average times during the day which young adults experience highs and lows of various activities. It assumes that you go to sleep about 11 p.m., sleep soundly, and waken at about 6 a.m. If your day runs earlier or later, the schedule is shifted by a corresponding amount.

About problems related with biological rhythm:


Jetlag is desynchronization in biological rhythm happens when crossed a number of time zones in an air plane, when we reach our destination, our internal clock and real timing are not synchronized.

For instance, a traveler from USA to Paris starts his vacation with four days of disorientation. Two hours after midnight, he will be sitting up in his bed looking for his coffee and croissants. Two hours past noon he will be ready for bed. His body will gradually shift to a new routine as melatonin secretion becomes adjusted so that the hormone signals begin arriving at their target neurons on Paris time.

15Figure 9: Graph and Information on how missed sleep effect sleep-wake cycle

WINTER BLUES (Winter Depression):

Symptoms of severe winter blues, or seasonal affective disorder (SAD) are depression and an overwhelming desire to sleep. This discomfort may result from a internal clock that is not synchronized with the changes in day length during winter (when days are shorter and nights are longer). Their symptoms worsen when they are given doses o melatonin. And they improve dramatically when they are exposed to intense light, which shuts down pineal activity.


Insomnia can be best defined as a condition of inadequate or poor quality sleep. It is characterized by the inability to fall asleep and/or the inability to remain asleep for a reasonable amount of time. Insomniacs have been known to complain about being unable to close their eyes or “rest their mind” for more than a few minutes at a time. It is a problem that affects a great many people. Besides some other reasons that can bring in insomnia, like depression or chronic anxiety, changes in one’s daily routine is the most common cause of insomnia. For instance, travel, starting a new job or a new semester, going into hospital, sleeping in a strange environment, moving into a new home, and anxiety over job interviews or tests are some possible causes of insomnia.


Our behavioral activities are governed by artificial cycles of light, whereas our physiological activities are governed by natural cycles. 18Therefore, modern life applications give us the chance to wake before dawn, or sleep late after sunset. When we stay much later on Saturday night, sleep much later on Sunday morning, we suffer the consequences on Monday morning when our biological clock is desynchronized.

Such minor problems about sleep is also very common among people. At least, we can say that many people claim that they don’t sleep enough or effectively. They claim mostly that the time they slept is less than needed. Here a question occurs. How can we realize a relationship between the number of hours slept and the soundness of the sleep? For average, a normal sound sleep shouldn’t be less than 6 hours and more than 9 hours. Which means that sound sleep doesn’t only have a minimum limit but also a maximum limit. Another major advice on sleep is to sleep at the same time every night which will regulate our internal clock, as sleep-wake cycle is a major issue of biological rhythm.

Therefore it can be hypothised that the number of hours slept should be parallel to our biological rhythm for us to have a sound sleep. Which means that the most effective night sleep would occur when sleep in a rhythmic structure every night and obey these hours in a fairly strict way.

It can be predicted that if we sleep parallel to our biological rhythm and therefore sleep in the number of average sleep hour, we would have the most sound and effective night sleep.

Method Development:

In order to do the investigation on the relationship between soundness of sleep and sleep hours, in the first step, one needs to have the data of how many hours slept. It seems quite easy as one can calculate this timing by recording the times of waking up and setting to sleep. The longer the duration of the record, the less error in investigation. 14 days (2 weeks) of recording will be used in this paper.

In order to relate our biological rhythm to the issue, one needs to set a standard for the internal clock. For these standards, the most useful and practical criteria would be a numerical average. Therefore the average of the hours slept will be essential for this investigation.

The soundness of the sleep is quite a difficult criteria to have a numerical value to show. Consequently the variable will be chosen from a very narrow range. As one wakes up, if they have slept soundly, he or she should get up without difficulty. If the difficulty occurs it would probably show itself with begging for some more minutes or not willing to get up. This will would bring in some time between the stimulus that makes the one wake (a bell ringing or someone calling the one to wake up) and getting up. The one can even fall asleep again, which will show how weak the sleep was. Therefore, the most practical criteria to determine the soundness of the sleep would be the number of minutes that pass after the bell rings (or being called to wake up) and before getting up. One can determine when the bell would ring and record the minutes that pass after that moment as soon as he or she gets up and record the data. As the habits of the one is effective we can take the most common timing as the habit and calculate the deflection in order to see if there has happened any problem with waking and use the percentage increase in this time to determine if it has become harder for the one to wake up and how much with respect to each other.

Therefore the method can be based on these two variables: number of hours slept and the minutes that pass between the stimulus to wake and getting up.


Within two weeks, each day, the time of waking is set with a bell, and recorded with the hours that one sets herself or himself to sleep. Also the times the one actually gets up is recorded.

Using these raw data, calculations will be done on number of hours slept. The average of these processed data will be used to determine the biological clock of the one. The deflection of each night from the average in particular will determine how parallel to his or her internal clock one slept.

The other calculation will be done on how many minutes passed after the waking stimulus and actual waking. These data will determine the soundness of the night sleep. The common habit on the issue will be determined and with respect to this value how harder it got to wake up will b e calculated.

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