Circadian Rhythm Essay

Circadian rhythms and biological clocks

by LW

Life begins, and ends. The passing of time is marked by many rhythms: that of our pulse, breathing, the rotation of the Earth. Sunspot activity occurs in a cycle of around 13 years. The solar system completes a circuit of our galaxy every 250 million years.

Within this, the sun rises and sets, and rises again, in a pattern that repeats itself on a daily, 24-hour basis. This 24-hour cycle is known as a biological clock, or circadian rhythm.
Living World was assigned the section dealing with circadian rhythms for the Time! Time! Time! exhibition at the National Museum of Emerging Science and Innovation (2003).

Before embarking on construction of the exhibit, we visited the laboratory of Yamaguchi University professors Kenji Tomioka (now at Okayama University) and Shinichi Inoue, where they showed us some of their fascinating material and experiments. The following is a report on what we found.

Rats are nocturnal. Kept in a cage, once night falls they will start to exercise busily on their treadmill.

The graph below shows that rat activity. The horizontal axis displays two days/48 hours of back-to-back data. The bold section shows rat exercise on the treadmill.
Look at the block from the top down to “a”. This shows that the rats are active at night, and do not spend much time on the treadmill during daylight.

From The Brain and Rhythm by Hiroshi Kawamura (Asakura Shoten, 1989)

The person conducting the experiment artificially rolled back the period of darkness by 15 hours starting at point “a”. It took around two weeks for the rats to shift the timing of their activity to coincide with the artificial nighttime.

What happens next is both interesting and important.
The rats’ habitat was made completely dark, with no night or day, starting at point “b”. Now you’d expect that losing any sense of night and day would disrupt the rhythm of the rats’ existence…but it didn’t. They retained the same pattern of existence for over a month. Is this a kind of inertia?
Or do rats possess an internal clock that marks time regardless of the movement of the sun or light and darkness?

Look closely, and you will notice that the period during which the rats are active gradually becomes skewed.

From The Brain and Rhythm by Hiroshi Kawamura (Asakura Shoten, 1989)

It seems living creatures have a self-regulating internal clock that marks the time in a fixed cycle regardless of whether it is light or dark, or the movement of the sun.

This has been found to be the case for most creatures on the Earth’s surface. But where exactly is this clock? Its location has finally come to light just recently.
The photo here shows a cross-section of a rat brain. Cells containing genes that govern the rat’s internal cycle have been found in the suprachiasmatic nucleus or SCN, the section of the brain indicated by the arrows.

From The Brain and Rhythm by Hiroshi Kawamura (Asakura Shoten, 1989)

A enlarged photo showing the cells in question. A gene mechanism controlling the rhythm of rise and fall in protein levels was discovered here.

From The Brain and Rhythm by Hiroshi Kawamura (Asakura Shoten, 1989)

When these cells are cut away and cultured separately, despite being severed from the living body, each individual cell operates on a cycle of approximately 24 hours. (Continuous record of circadian rhythm in separated SCN [Inoue et al, 1981])

But all this deals with rats (chosen because they are easy to use in experiments to represent all mammals). But what about humans?

R. Wever, The Circadian System of Man

The following is taken from The Circadian System of Man, by researcher R. Wever. Wever created a underground living environment shut off from contact with the outside world, and with no pattern of light and dark, and used students and other volunteers to conduct experiments (1969) in which he investigated what would happen to people’s patterns of waking and sleeping.

R. Wever, The Circadian System of Man

This graph shows the data from one of Wever’s subjects. An average cycle of around 25 hours became evident.
Of course there are always exceptions, and there were people who suddenly moved to a 33-hour cycle from the second week, or some who sped up and fell into a cycle of 16 hours.

In any case, it seems that like rats, we humans also possess an internal clock that marks time regardless of light and darkness. That clock moreover is largely similar to the 24 hour (23 hours 56.41 seconds to be precise) cycle on which the Earth rotates, just around one hour out.

From The Brain and Rhythm by Hiroshi Kawamura (Asakura Shoten, 1989)

This circadian or daily rhythm has been confirmed not only in mammals such as rats and humans, but in plants, molluscs, insects, amphibians, birds, in fact almost all life forms found on the surface of the Earth.
Some have cycles longer than 24 hours, some shorter. Most cycles however are clustered around four hours either side of 24 hours. There are also life forms living in places sunlight does not reach, such as deep in caves and at the bottom of the ocean, for which no such rhythm can be confirmed.

Thinking about it, the Sun is the source of energy for all life on Earth.
Solar energy that reaches the surface of the Earth is first of all converted by plants into organic material, and other organisms use that material, living and growing interdependently.
The rhythm of the biological clock is one that must have taken the life forms dependent on the Sun a very long time to develop.

Our next task was to build an exhibit using this background information. To A DAY/Rhythms of Life (Time! Time! Time!, MeSci 2003)

by LW 19 03, 2003

← 2003-03-192003-03-19 →

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.

This is an essay answer taken from the paper 2 ebook for AQA psychology A level students studying paper 2 as part of their AS and A level psychology course (new specification). You can download all the possible essay questions and model answers in this paper 2 ebook by clicking the image cover at the top.

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