Ultradian Rhythm Essay Format

Zucker’s study where he damaged the SCN in rats to disrupt circadian rhythms was an animal study and may not apply to humans due to differences in anatomy. Therefore it may lack external validity and generalisation in humans. There are also ethical concerns when it comes to intentionally harming such animals although others may argue the benefits gained in understanding animal biology may lead to further understanding of humans. Such studies are typical of the biological approach to understanding human behaviour. They propose behaviour can be explained due to biological structures in the brain or hormonal activity. In truth our behaviour is much more complex and not so deterministic as such biological explanations propose. “Nurture” is evidently a strong factor too with environmental influences and exogenous zeitgebers clearly having a strong role in overriding internal biological clocks to some degree. On the other hand Miles et al demonstrated how a blind man who had a circadian rhythm of 24.9 hours struggled to reduce his internal pace no matter what exogenous zeitgebers were used highlighting some biological clocks may be more ingrained and not influenced.

The SCN is evidently not the only biological clock as other studies have shown that there are other oscillators in the body that appear to regulate biological rhythms through other means (temperature, light penetrating other parts of the body) and explaining circadian rhythms as simply dictated by the SCN and pineal gland connection is oversimplifying the workings of human biology which is far more complex.

Understanding circadian rhythms has real world applications particularly in the field of Chronotherapeutics. This is the study of how timing affects drug treatments and as the circadian rhythm affects digestion, heart rate and hormones among other functions, this can be taken into account when consuming drugs. For instance medicine that affect certain hormones may have no effect if taken when the target hormone level is low but more effective if taken when they are high. Aspirin for example is most effective in treating heart attacks and most effective if taken in the late evening as most attacks occur in the early hours of the morning.

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<a href=”https://www.loopa.co.uk/circadian-rhythm-biological-rhythms-aqa-psychology/”>Circadian Rhythms – Biological rhythms</a>

The following is an analysis of the relation of biological rhythms to mood disorders. The goal is to identify and describe the various cycles that exist within the body to maintain a harmonious rhythm, including circadian rhythms, chronobiological factors, and biopsychosocial rhythms, specifically their relation to the seasons. Once an understanding these rhythms has been gained it is possible to determine external factors, which act to significantly, disrupt their harmony.

Finally by relating the function of biological rhythms to the psyche, specifically their impact on mood and personality it is possible to show how a shift can result in a disorder. In addition to reviewing the stated relationships this analysis will also cover methods of preventing and treating mood disorder that result from changes in biological rhythms.

Biological Rhythms

By taking the two words and breaking them down into their separate meanings a definition of biological rhythms evolves as, life which involves a recognizable pattern of change over time. There are two major categories of biological rhythms, endogenous, and exogenous. Endogenous rhythms come from within and are regulated by the organism itself, for example the body temperature cycle. Exogenous rhythms are the result of external factors, such as a change in the seasons, or the transition from day to night. Environmental stimuli that help to maintain these cycles are called zietgebers, which comes from German and translates as "time givers." Zietgebers include sunlight, noise, food, and even social interaction, all cues that help the biological clock maintain a 24-hour day.

There are four categories of biological rhythms that extend beyond just classifying them based on internal and external sources. This system maintains that criteria, but extends to include the duration of the cycle as a defining factor. The resulting categories are circadian rhythms, diurnal rhythms, ultradian rhythms, and infradian rhythms.

Circadian Rhythms

Circadian rhythms are defined as an endogenous rhythm pattern that cycles on a daily (approximately 24 hour) basis under normal circumstances. The name circadian comes from the Latin circa dia, meaning about a day. The circadian cycle regulates changes in performance, endocrine rhythms, behavior and sleep timing (Duffy, Rimmer, & Czeisler, 2001). More specifically these physiological and behavioral rhythms control the waking/sleep cycle, body temperature, blood pressure, reaction time, levels of alertness, patterns of hormone secretion, and digestive functions. Due to the large amount of control of the circadian rhythm cycle it is often referred to as the pacemaker.

Two specific forms of circadian rhythms commonly discussed in research are morning and evening types. There is a direct correlation between the circadian pacemaker and the behavioral trait of morningness-eveningness (Duffy et al., 2001). People considered morning people rise between 5 a.m. and 7 a.m. go to bed between 9 p.m. and 11 p.m., whereas evening people tend to wake up between 9 a.m. and 11 a.m. and retire between 11 p.m. and 3 a.m. The majority of people fall somewhere between the two types. Evidence has shown that morning types have more rigid circadian cycles evening types, who display more flexibility in adjusting to new schedules (Hedge, 1999). One theory is that evening types depend less on light cues from the environment to shape their sleep/wake cycle, and therefore exhibit more internal control over their circadian rhythms.

Diurnal Rhythms

Diurnal rhythms are an extension of circadian rhythms. Simply put the diurnal cycle is identical to the circadian cycle, with the one additional corollary that it must be in sync with the day and night cycle. In other words, for an individuals circadian rhythms to become diurnal that subject must be awake and functioning normally during daylight hours and sleeping during night hours on a fairly consistent basis. Note that it is possible to have a circadian cycle without being diurnal but not visa versa.

Ultradian Rhythms

Ultradian rhythms are defined as an endogenous rhythm pattern that occurs on a shorter time scale than circadian rhythms. As a result of the brief cycle time the frequency of occurrence is much higher. A prime example of an ultradian rhythm is feeding patterns. For the average person this cycle repeats about 3 times a day. Unlike diurnal rhythms ultradian rhythms are share no overlapping relationship with circadian rhythms.

Infradian Rhythms

Infradian rhythms are defined as an endogenous rhythm pattern that has a cycle duration longer than circadian rhythms, that is more than 24 hours per cycle. Due to the longer time frame for each cycle the frequency of occurrence in these cycles is lower than that of the circadian rhythms. The female menstrual cycle is an example of an infradian rhythm. It is a cyclical biological event that occurs in a fairly regular pattern on a monthly basis. Similar to the ultradian cycle the infradian rhythms are not directly linked to circadian and diurnal rhythms. Additionally these rhythms are not believed to be influenced by the daytime/nighttime schedule or changes in available natural light due to the fact that infradian rhythms occur in an unrelated time pattern.

By reviewing the function of these various biological rhythms it is clear to see that a majority of our physiological systems, and behaviors are directly controlled or influenced by these patterns. The question that arises is what external factors help to establish biological rhythms? Additionally what happens if the cycle is altered by a change in those external factors? In the following sections the answers to those questions will be explored, specifically as they related to mood disorders.

Factors Influencing Biological Rhythms

The major biological rhythms discussed thus far are primarily endogenous, or internal in nature. That is to say they originate from within the organism. Theoretically if we could isolate an individual from all outside influences it would be possible for their biological rhythms to normalize and occur on a strict schedule. In reality, however, we are subjected to a wide variety of changing stimuli all the time. It is not surprising then, that some of these changing stimuli have been shown to influence the regularity of our biological rhythms.

The Circadian Clock

In the physical sense circadian cycles are controlled by the circadian clock, a cluster of approximately ten thousand nerve cells located on the suprachiasmatic nuclei (SCN) found on the hypothalamus in the brain. The circadian clock's primary function is to interpret external changes of light and darkness, as well as social contact, in order to establish diurnal rhythms. It is not uncommon for the circadian clock to be disrupted temporarily, events such as changes in work schedule from day to night, changing time zones (also referred to as jet lag) and to some extent old age can impact the consistency of circadian rhythms.

Influence of Light

Light is an important factor for maintaining biological rhythms. The circadian clock relies heavily on changes in light to determine transitions from night to day. During periods of darkness the SCN clock sends out the hormone melatonin, which induces sleep. It is plain to see how changing work schedules from the day shift to the night shift would create the need to reverse this process, which takes time and will in turn disrupt normal rhythmic patterns. Circadian rhythms in shift workers were shown to adjust an hour or two per day (Hedge, 1999). This means that it could take over one week for an individual to fully adjust to an 8-hour shift change.

Another major disruptive factor related to the circadian clock's interpretation of light is season changes. During winter months there are fewer daylight hours, as a result the level of melatonin secretion increases along with the number of hours of darkness.

Disruptions in Feeding Cycles

Previously it was mentioned that ultradian rhythms regulate short-term patterns, such as feeding cycles. Crystal (2001) has shown that behavior related to feeding is indeed caused by a biological rhythm and not external environmental cues. In order to do so he first showed that there was an increase in anticipatory activity before food was presented when a regular feeding schedule was used on rats. The schedule was made to extend beyond a 24-hour period in order to rule out time of day and environmental cues such as sunlight. It was shown that the anticipatory activity still increased even when food was withheld. Additionally the rats in the study showed a tendency to gradually adjust to changes in the feeding pattern, much as shift workers gradually altered their sleep/wake patterns after a change in working hours.

Although it has been shown that the circadian clock, located on the SCN is apparently not responsible for regulating feeding intervals, the studies conducted on rats showed that rats on a feeding cycle between 22 and 26 hours (the circadian range) where better able to anticipate food arrival than rats fed on schedules outside this range, including 7 and 34 hour intervals. Based on this information it can be shown that a drastic change in meal times would have a similar effect on the ultradian rhythms as a change in work shift has on circadian rhythms.

Caffeine's Influence on the Circadian Clock

In addition to these major influences there are a variety of other environmental factors that may have an impact on biological rhythms. A stand out among those currently being researched is caffeine. A series of experiments on caffeine revealed differences in the effects of the drug depending on time of day. In the morning caffeine was shown to hinder low impulsives while helping high impulsives, while the opposite was true in the evening (Revelle, Humphreys, Simon, & Gilliland,1980). This finding suggests that low impulsives and high impulsives differ in the phase of their diurnal rhythms, which resulted in a difference in the effects of caffeine.

By establishing an understanding of the environmental factors that influence biological rhythms it is possible to begin drawing connections between the resulting shifts and mood disorders. It is important to note also that a significant shift must occur before a mood disorder, or any other mental or physical health problem will develop. Most living things experience fluctuations in waking/sleeping cycles and feeding cycles for example, however the amplitude of those fluctuations is generally small enough that normal rhythmic cycles can adapt without a detrimental impact on health. The question that arises is when do health problems, specifically mood disorders; develop as a result of shifts in biological rhythms, and what combination of environmental factors leads to those shifts? Furthermore, what can be done to prevent mood disorder causing shifts, and how can existing mood disorders of this form be treated?

Mood Disorders and Biological Rhythms

Mood disorders are characterized by opposite polar moods: depression, which involves extreme feelings of sadness and dejection, and mania, which involves unrealistic feelings of excitement and joy. There are a variety of unipolar mood disorders, which involve mania or depression, and bipolar disorders, which are characterized by both mania and depression.

Sleep and Depression

As previously discussed the circadian clock is responsible for controlling sleep patterns. Melatonin secretion from this region of the brain actually induces sleep. Commonly depressed patients experience a wide variety of sleep disorders. It should come as little surprise then that there is a connection between disruptions of the circadian cycle and depressive disorders. Generally a decreased amount of deep sleep per night comes just before the onset of depression. Therefore a drastic change in sleep schedule caused by extensive occurrences of jet lag, or multiple shift changes may result in a disruption of circadian rhythm function. In these instances it is possible for the circadian clock to induce REM sleep 15 to 20 minutes earlier in the sleep cycle, resulting in decrease in the amount of deep sleep, and ultimately leading to the beginning stages of depression (Butcher, Mineka, & Hooley, 2004).

In order to help prevent disruptions in the circadian sleep cycle it is important to maintain a regular sleep schedule, which includes retiring and waking at approximately the same time each day, and sleeping a consistent number of hours each night. This is especially important for people with morningness tendencies because their circadian cycles are less adaptable to changes in behavior.

Seasonal Affective Disorder

In recent years psychologists have recognized the impact of seasonal changes on mood and behavior. Seasonal affective disorder (SAD) is a unipolar mood disorder in which patients are highly responsive to the total amount of light available in the environment (Oren & Rosenthal, 1993). Individuals who suffer from seasonal affective disorder show signs of depression during the fall and winter months when there are fewer hours of sunlight each day. Disturbances in mood are the main psychological component of seasonality (Ennis & McConville, 2004).

Persons suffering from seasonal depression generally show an increase in appetite and hypersomnia, which oddly is opposite of the behavior normally associated with most other forms of depression. This behavior is consistent with research conducted on animals and may be related to baser survival instincts. The explanation behind this theory is that like some animals people may have a natural tendency towards increasing fat stores in the body during the winter, as well as sleeping more often in order to preserve energy levels.

Several more recent studies suggest that suffers of seasonal affective disorder display disturbances in their circadian cycles, as indicated by less consistent rhythm patterns. A common therapy used to treat seasonal affective disorder is light exposure therapy (Oren & Rosenthal, 1993). Though the effects of light exposure are not completely understood it has been shown that the presences of either natural or artificial light seems to work towards correcting circadian disturbances caused by seasonality.


There are four types of biological rhythms that regulate cycles within the body. The primary type, circadian rhythms, controls performance, endocrine rhythms, behavior and sleep timing, and is regulated by the circadian clock, a collection of nerves located on the hypothalamus. Diurnal Rhythms are a specialized form of circadian rhythms, which are closely synchronized with day and night cycles. Both cycles have a duration of approximately 24 hours. Ultradian rhythms are biological rhythms, which operate on a shorter time scale than circadian rhythms, feeding schedules for example. Infradian rhythms are those with cycles longer than 24 hours, the most commonly studied example is the human menstrual cycle.

Although all of these biological rhythms are controlled internally there are a number of external factors that are capable of influencing their regularity. Some of the most prominent examples are exposure to light, specifically the changes caused by seasonal transitions, alterations in work shift which change sleeping schedules, jet lag, and caffeine. With the exception of light affects the other influencing factors cause sleeping patterns to change. Due to the fact that circadian rhythms can only shift one to two hours each day drastic changes in sleep patterns can have a detrimental effect on the circadian clock.

Seasonal changes cause an alteration in the amount of light that individuals are exposed to. During the months where the days are shorter, primarily in the winter, circadian patterns are disrupted. The reason is that the circadian clock is programmed to release melatonin to induce sleep, a function that is initiated by darkness. Because the sun sets earlier in winter months this reaction begins occurring earlier in the evening, which results in a disrupted sleep pattern, a common problem for depressed patients. Individuals with Seasonal affective disorder are more likely to experience the affects of this change, and are prone to an increased amount of sleep, known as hypersomnia, and an increased appetite.

There is still a great deal that is not known about the relationship between biological rhythms and mental and physical health disorders, however there is enough existing evidence to support further study in this field. By gaining a better understanding of the rhythms and environmental factors that influence them it is possible to begin making connections to mood disorders. Once the link can be traced it is possible that new treatments may develop which are designed to correct disruptions in biological rhythms, or perhaps even prevention methods, which help to avoid major disruptions.


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