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The circadian rhythm: A brief look at our internal clock

I recently attended a lecture on Biorhythms by one of the 2012 Canada Gairdner Awardees, Dr. Jeffrey C. Hall, Professor Emeritus of Biology at Brandeis University (USA), and decided to briefly discuss the circadian rhythm.

So what is the circadian rhythm? The circadian rhythm is a near 24 hour rhythm maintained by the suprachiasmatic nuclei of the hypothalamus and is considered to be our physiological "master clock". The hypothalamus is located approximately in between and at the level of the eyes and a couple inches inside the head from the front.  The circadian rhythm is called the master clock because it dictates the cyclicity of a number of other hormones in the human body.

The "pacemaker" of the circadian rhythm receives inputs from external (light, social and physical activity, sound) and internal (melatonin) signals which drive the outputs, such as behavior and psychology (Reid et al). According to Doherty and Kay, "Understanding how the clock controls downstream processes [signal transduction] has widespread clinical impacts, as it affects many diseases and biological processes including immune responses, metabolism, and aging".

Interestingly, the circadian rhythm is not particularly fixed to a rhythm and it can be reset relatively easily. As mentioned above, the inputs are the main drivers of the rhythm with light and melatonin being the most important of these inputs. According to Reid et al, "the timing, intensity, duration, and wavelength of light determine the direction and magnitude of light-induced phase shifts of the circadian clock. For example, bright light exposure in the evening results in phase delays (shifts the biological clock later), whereas light exposure in the early morning results in phase advances (shifts the biological clock earlier)." Melatonin also plays a key role and has the opposite effect of light with regards to phase shifting (Reid et al). Indeed, it can be given as a treatment for some sleep disorders.

Though leaps have been made in our comprehension of the circadian rhythm since the study of chronobiology began around the 1950s, much remains elusive. Leaders in the field such as Dr. Hall continue to work in the lab using Drosophila species and hopefully will continue to shed light on how the internal clock functions. Research in the field is particularly important because of the interconnection between much of how the body functions and the circadian rhythm, and certainly also for those who suffer from circadian rhythm sleep disorders.

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