The circadian rhythm is better known as your body clock, and is responsible for regulating many functions in the body from sleep, hormone production and even physical performance. It is integral to how the body works; ever felt jet lagged, or exceptionally groggy from waking up in the dark? This is because the circadian rhythm has been disrupted.

This article will highlight some key areas in the circadian rhythm, explore the reason for it existing, explain what key factors regulate your circadian rhythm and the effects disruption can have on your health.

 

Circadian Rhythm key points

Bellow are some key points which have been recorded in a normal human circadian rhythm. The times are approximate, and will vary slightly from person to person, but generally speaking, this is how it is structured:

• 2am – This is the point of deepest sleep.
• 4am – 5am – Body temperature drops to its lowest point. Sleep is still very deep.
• 6:30am – 7:30am (typically with sunrise) – Sharp spike in blood pressure, followed by a reduction in melatonin secretion (a hormone which makes you feel tired). This causes sleep to lighten, metabolism to slightly increase, and you may naturally wake up around this period.
• 8am – 9am – during this period of time a bowel movement is likely, and testosterone levels are at their highest of the cycle.
• 10am – Highly alert, the mind is very active around this time.
• 2pm – 4pm – During this period your hand eye co-ordination and reaction time are at their best.
• 5pm – Cardiovascular system is at its most efficient, and possibly as a result, muscle strength is also at its highest.
• 6pm – 7:30pm – Blood pressure and body temperature are at the highest of the cycle.
• 7:30pm – 9:30pm (typically with sunset) – Melatonin secretion is stimulated, causing to the feeling of tiredness.
• 10pm – Bowel movement is slowed down until early hours of the next morning.

 

Purpose of circadian rhythm

Circadian rhythms have been well recorded in humans, animals, plants and micro-organisms for hundreds of years, yet the actual purpose is very difficult to prove, and so the accepted purpose of our circadian rhythms is largely from educated guesses. As these rhythms have been observed in such a variety of organisms (to various complexities), it is accepted that they have been a part of early evolution, and have developed in complexity, as we as organisms have developed in complexity.

Circadian rhythms allow our body to anticipate and prepare for regular environmental changes (such as sun rise) and ensure we are biological ready for what is coming. This ensures our body will perform at its best, when it is most likely needed to, and is ready for rest and repair when is is likely to be able to. An example of these regular environmental changes are day length and temperature, which, although do vary, cycles over a 24 hour period. During the night, we sleep (the purpose of which complex, but includes for protection, energy conservation, and body repair), and so melatonin is produced to make us tired as the sun goes down. We have evolved to sleep at night because we are a very visually reliant species, and as the light goes, historically, we would be useless. Other animals which, are more active at night may have an opposite circadian rhythm response.

 

Disrupting your circadian rhythm

The circadian rhythm can be disrupted (by working night shifts for example), which can alter the way it influences our body.

Without the typical stimuli (heat and light) your circadian cycle will continue to run normally for some time, however, if you go in an extreme controlled environment you will go into a ‘free running cycle’, or rather a non-existent cycle. This has been observed in controlled situations with humans, and in Arctic animals during periods of no exposure to light.

Our circadian rhythm can be very easily disrupted by exposure to artificial light when there is no natural light, and this is a growing concern for many in developed countries with the increased use of computers, artificial lights and even mobile phones late at night. This can cause production of melatonin to be inhibited, which can result in disrupted sleep. This in turn, can reduce growth/repair (which occurs in your sleep) and can prevent other key points in the circadian cycle occurring, meaning physical and mental performance can be reduced the following day. Disruption of your circadian rhythms is thought to be responsible for a condition called seasonal affective disorder (SAD), which causes low moods (in extreme circumstances can cause sever depression) and an increased appetite. This is most common during winter months, where many people wake up in the dark to go to work, but can also be caused by being exposed to light late at night. Many people who travel will experience ‘jet lag’, which is characterized by tiredness and low/ poor mood, and is a result of disrupting their circadian rhythm by passing through time zones.

The circadian rhythm is regulated by specialized cells in the hypothalamus, which are directly connected to the eyes. This helps to explain how light regulates circadian rhythms, and shows how they can be linked to hormone regulation (hypothalamus produces many hormones), which controls appetite and mood (as well as a number of other functions). Destruction of these cells has been shown to the natural sleep-awake cycle of the body.

 

Summary

Circadian rhythm (your body clock), is an important part of your basic biology, and is integral to overall health. It is largely regulated by sunlight, but other factors such as temperature play a role in its regulation as well. Disruption of the circadian cycle often has a negative effect on mood, appetite, growth/repair and brain function, but these effects are rarely sever, and never life threatening.
 
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Main image courtesy of Clint Gardner