Biological Rhythm

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Time Zone Changes

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Circadian Clock

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Biological Calendar

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Circadian Rhythm Triggers

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Wake Periods

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Rest Periods

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Biological Rhythms

Have you ever wondered why you are able to wake just prior to your alarm clock?  Have you wondered why your body seems to know exactly when its dinner time even before you do?  Did you also know that you can use ‘Jet-Lag’ as an excuse for anything from fatigue to memory loss and clumsiness, even if it’s only a few hours difference? Daylight Savings Time counts as well!

Interestingly, our body is equipped with rhythms that can tell us when we’re hungry or tired right on cue; even if we were to be locked in a room without windows are access to the outside world.  Scientists are still working on these biological rhythms, or (clocks) to this day, and it is a very popular topic among many researchers and psychology students with new studies coming out regularly.

What are Biological Rhythms?

A Biological Rhythm is a natural cycle within the body that is either based on internal factors or external factors, also known as endogenous or exogenous.

Our endogenous rhythms, controlled internally from within the body are responsible for automatic functions such as body temperature, female menstrual cycle, cell division, hormonal release, heart rate, breathing and REM sleep.  Endogenous rhythms have what is termed as ‘Endogenous Pacemakers’ or ‘internal biological clocks’ which control our rhythms.  Our main pacemaker is called the ‘Suprachiasmatic Nucleus’ (SCN) which also happens to be the favorite part of the brain of any neuroscientist who studies Biological Rhythms.

The Suprachiasmatic Nucleus is a cluster of about 10,000 cells within an area of the brain called the hypothalamus and controls circadian rhythms of various body functions.  These cells basically control every other cell in the human body. 

The SCN can detect sunlight through the retina and will alter the sleep-wake cycle accordingly.  This cycle works on the pineal gland from a nerve pathway from the retina to the hypothalamus. The SCN initiates communication with other parts of the brain and can alter the release of a hormone called melatonin which induces sleep.  The Suprachiasmatic Nucleus will also delay this release of melatonin until darkness sets in outside.  It is no wonder this area of the brain is so highly researched by scientists!

Our exogenous rhythms are controlled by external factors or stimuli such as daylight, noise and social interaction.  These stimuli, also known as Zeitgebers, are cues from the external environment and vital to the resetting of the biological rhythms in an organism. 

Our Four Biological Rhythms

  1. Circadian Rhythms

An internal timing mechanism based on a 24 hour internal cycle.  When it comes to circadian rhythms, our bodies naturally adjust to the two basic environments being day and night.  Circadian rhythms continue to run by external environmental cues called ‘zeitgebers’ which fall into the category of daylight, noise and social interaction. 

However, studies have proved that during an absence of zeitgebers the circadian clock will continue to run on its own internal biological clock.  The circadian rhythms that run without the external cues of zeitgerbers, are called ‘free-running rhythms’, and will continue to run solely on the internal clock.

The suprachiasmatic nucleus has also been found to control the internal body temperature via the light / dark 24 hour cycle.  These changes in temperature have significant effects on the circadian clock also contributing to many cellular responses. This circadian rhythm is affected by postural changes, eating, sleeping and physical activity. 

  1. Diurnal Rhythms

This biological rhythm is identical to the circadian rhythm, except that it must be in sync with the day-night cycle, and wake time must be during the day, and sleep time must be during the night.

  1. Ultradian Rhythms

This biological rhythm runs on a much shorter time frame than the circadian and diurnal rhythms.  An example of this rhythm is the feeding cycle such as breakfast, lunch and dinner.  Ultradian cycles can run as frequently as every three hours.

  1. Infradian Rhythms

This cycle has a much longer time span as it is based on monthly intervals.  An example would be a woman’s menstrual cycle.

The Role of Melatonin

This natural hormone produced by the pineal gland will automatically detect when sleep is to begin once it darkens outside.  The pineal gland will begin to release the hormone called melatonin to induce sleep, and as sleep is underway the melatonin will continue to be released by instruction from the SCN until daylight, when melatonin ceases to be released, and the pineal gland is basically turned off again. A key factor to keep in mind with the release of melatonin is that bright indoor light can also trick the suprachiasmatic nucleus (SCN) into believing it is daylight, and the release of melatonin by the pineal gland will stop.  In reverse, a dimly lit room can also bring on the release of melatonin.

The Honeybee and Circadian Rhythms

The honeybee is a prime example of the nature of biological rhythms, and very similar to that of humans, the honeybee also has internal biological clocks.  Without the honeybee’s biological clock, they would not know when to pollinate, which is an incredibly time meticulous activity.  

What Factors Disrupt Biological Rhythms?

Jet lag, also known as physiological desynchronization caused by trans-meridian travel through time zones is a proven phenomenon of the circadian rhythm.  Individuals that travel tend to struggle with fatigue and daily functioning even in the event that the time difference is within a few hours. Shift work will also cause the same symptoms as it can take up to 8 hours for an individual to adjust to a shift change from day to night or vice-versa. 

Drug use such as ecstasy has also been found to disrupt the natural biological clock as well.  Scientists have found that the drug interferes with the release of serotonin, and thus presenting symptoms similar to jet lag.  The drug will interfere with the natural workings of the circadian clock with problematic results in which the internal clocks may not be as affected by external stimuli, and thus unable to be reset according to environmental cues.  This damage can result in permanent dysfunction of serotonin pathways resulting in depression and mood disorders.

Biological Rhythm and Disorders

When the internal biological clock is disrupted by various means, it can result in a range of mood disorders.  Studies have found that people with frequent travel schedules such as pilots and flight attendants are more prone to depression and fatigue.  The continual change in REM sleep will disrupt the sleep cycle resulting in less deep sleep resulting in possible depressive symptoms.  Seasonal Affective Disorder is also a common result of circadian rhythm disruption.  Caused by the reduction of sunlight during winter months and altering serotonin release, Seasonal Affective Disorder will cause depressive symptoms.  Scientists have however found that sufferers of this disorder can use a timer with UV lights that are set to turn on just prior to awakening, providing the needed light for the circadian rhythm. This has proven to help fight Season Affective Disorder.  Dysregulation of the circadian rhythm has also found to be correlated with bipolar disorder.  The zeitgeber of artificial light has been shown to be very effective in the treatment of biological rhythm disorders and their consequences.

Long term disruption of biological rhythms have shown a positive causal effect with such illnesses as mood disorders, gastrointestinal problems, glucose intolerance, hypertension, cancer, cardiovascular disorders and immune deficiency disorders and a possible shortened life span.

Biological Rhythms and healthy maintenance of regular sleep and eating schedules is vital to your health, probably more so than many realise.  Scientists will continue to study biological rhythms with much passion as they search for answers as to how the 10,000 cells of the suprachiasmatic nucleus keeps track of the 24 hour cycle so well, and how those cells can control almost every function in the human body.  They will continue to find ways to further improve our understanding and functioning of our circadian rhythms, resulting in more answers and better health for everyone. 


  14. Pinel, J. P. (2011). Biopsychology (8th ed.). Boston, MA: Pearson.




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