The Internal Body Clock

Natasha Lynch

Evidence that led to the discovery of the Circadian Rhythm and the endogenous pacemaker.

The first evidence of an internal body clock came not from our species but from a particular plant. In 1729 two scientists discovered that plants generate their own internal time. They focused on the plant Mimosa Pudica, this was because at night the leaves would collapse down almost as though they had wilted, but the next day the leaves would open again like an umbrella. This behaviour would repeat every morning and every night. Originally it was believed that this behaviour was due to the rising and setting of the sun. A researcher named Mairan completed a study where he placed a Mimosa Pudica in a sealed box for 24 Hours, taking away the natural cycle of light and dark of the earth. It was found, despite no longer having the influence of light and dark, the plant still behaved the same, the leaves would expand during the day and retract during the night even without the natural signal of the sun. This was an important discovery into the discovery of the circadian rhythm as Mairan had shown that a living organism kept its own time and that it was not due to the light and dark cycles of the earth. The findings indicated the plant must have an endogenous pacemaker which would generate its own internal clock.

Despite this discovery, it took more than 200 years to prove that humans had a similar mechanism. Two researchers, Kleitman and Richardson, took a trip to the Mammoth cave for 6 weeks. They went so deep that they had no implication of the time of day outside. The two researchers took a range of measuring devices to measure their waking and sleeping rhythms. They lasted 32 days in complete darkness and made 2 incredibly important discoveries, first that humans (like the Mimosa Pudica) generated their own endogenous pacemaker and circadian rhythm, even in the absence of light and dark cycles. This means both researchers had a predictable and repeating pattern of being awake (15 hours) and being asleep ( 9 hours). It was also found that the endogenous pacemaker was longer than 24 hours. This, therefore, suggests that the light and dark cycles of the earth are responsible for regulating the circadian rhythm to exactly 24 hours. More recently it has been discovered the circadian rhythm is around 24 hours and 15 minutes. 

Many other research has taken place similar to Kleitman and Richardson's study all which found that circadian rhythm was longer than 24 hours. I feel these two pieces of research were vital in the discovery of the internal body clock- the circadian rhythm and the endogenous pacemaker. 

Thanks for reading 

https://www.sleepfoundation.org/articles/what-circadian-rhythm
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1369194/

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