How does our body maintain precise timing ?
In the last article, we got introduced to what is circadian rhythm. This 24-hour tempo, controls our timed preferences for eating, sleeping, moods, emotions, our core body temperature, metabolic rate and release of numerous hormones.
Will our circadian rhythm go haywire in absence of sunlight ?
Long before we discovered this biological pacemaker, an ingenious experiment in 1729 by French geophysicist Jean-Jacques d’Ortous de Mairan discovered the very first evidence that plants generate their own internal time. De Mairan was studying the leaf movements of a species that displayed heliotropism: when a plant’s leaves or flowers track the trajectory of the sun as it moves across the sky during the day. De Mairan was intrigued by one plant in particular, called Mimosa pudica — at night, the leave collapse down as though they had wilted and at the start of the following day, the leaves pop open once again like an umbrella, healthy as ever. He wondered what would happen if the plant was cut from sunlight altogether. Will the leave still go through the cycle of opening and closing it’s leaves ? He kept the plant in a sealed box completely cut-off from sunlight.
Despite being cut off from the influence of light, leaves opened at the start of the day and then collapsed at night fall. It was a revolutionary discovery: de Mairan had shown that a living organism kept its own time, and was not, in fact, slave to the sun’s rhythmic commands. Somewhere within the plant was a twenty-four-hour rhythm generator that could track time without any cues from the outside world, such as daylight. The plant didn’t just have a circadian rhythm, it had an “endogenous,” or self-generated, rhythm.
Plants seemed to have their own biological clock.
Two hundred years later it was found that humans have a similar, internally generated circadian rhythm. In 1938, Professor Nathaniel Kleitman at the University of Chicago, accompanied by his research assistant Bruce Richardson, did an interesting experiment.
Kleitman and Richardson took a trip for 6 weeks into Mammoth Cave in Kentucky, one of the deepest caverns on the planet — so deep, in fact, that no detectable sunlight penetrates its farthest reaches. They were trying to answer a simple question : When cut off from the daily cycle of light and dark, would their biological rhythms of sleep and wakefulness become completely erratic, or would they stay the same as those individuals in the outside world exposed to rhythmic daylight? This experiment resulted in two groundbreaking discoveries — The first was that humans, like de Mairan’s heliotrope plants, generated their own endogenous circadian rhythm in the absence of external light from the sun. The second was that the inner clock cycles were not precisely twenty-four hours in length, but consistently and undeniably longer than twenty-four hours. Subsequent research has found out that, the average duration of a human adult’s endogenous circadian clock runs around twenty-four hours and fifteen minutes in length.
We routinely experience light from the sun that methodically resets our inaccurate internal timepiece each and every day, “winding” us back to precisely, twenty-four hours.
But just how our internal circadian biological clock worked remained a mystery !!
Identification of a clock gene
During the 1970’s, Seymour Benzer and his student Ronald Konopka asked whether it would be possible to identify genes that control the circadian rhythm in fruit flies. They demonstrated that mutations in an unknown gene disrupted the circadian clock of flies. They named this gene period. But how could this gene influence the circadian rhythm?
In 1984, Jeffrey C. Hall, Michael Rosbash and Michael W. Young , who were also studying fruit flies, aimed to discover how the clock actually works. They succeeded in isolating the period gene. They went on to discover that PER, the protein encoded by period, accumulated during the night and was degraded during the day. Thus, PER protein levels oscillate over a 24-hour cycle, in synchrony with the circadian rhythm.