Circadian rhythms

life-style Aug 5, 2021
Photo by Barth Bailey / Unsplash

If we observe, certain common natural things like feeling hungry, sleepy etc., appear at the same time every day. Similarly, birds return to their nest at dusk and they wake up at dawn regularly every day.  Several behavioural biological activities such as the nesting behaviour of birds in spring and migration of certain bird species in autumn, leaf movement, photosynthetic activity, growth, germination, and flowering of plants also appear at regular intervals of time.

Photo by Elena Mozhvilo / Unsplash

Such behavioural biological activities that occur at regular intervals of time are known as Biological Rhythms or Biorhythms.  The time interval between these activities may vary from minutes to years depending on the nature of the activity and the species.

Such regularly occurring biological rhythms, showing a periodicity of 24 hrs. are known as Circadian Rhythms or Diurnal Rhythms.
The term circadian is derived from the Latin, in which circa, meaning "around" (or "approximately"), and diēm, meaning "day”.
The rhythms are linked to the light–dark cycle. Circadian Rhythms are widely observed in almost all living organisms.

The earliest circadian process reported dates back to the 4th century B.C.  Androsthenes, a ship captain under Alexander the Great, described diurnal leaf movements of the tamarind tree.
The mention of circadian or diurnal processes in humans dates to around the 13th century, in Chinese medical texts.
French scientist Jean-Jacques d'Ortous de Mairan in 1729, observed an endogenous circadian oscillation and it is the first recorded observation. He observed 24-hour patterns in the movement of the leaves of the plant Mimosa pudica
In 1896, Patrick and Gilbert observed the sleeping pattern and stated that a prolonged period of sleep deprivation may cause increase and decrease of sleepiness with a period of approximately 24 hours.  
In the early 20th century, circadian rhythms were noticed in the rhythmic feeding times of bees.
Extensive experiments were done by Auguste Forel, Ingeborg Beling, and Oskar Wahl to see whether this rhythm was due to an endogenous clock.
In 1935 two German zoologists, Hans Kalmus and Erwin Bünning independently discovered the existence of circadian rhythm in the fruit fly Drosophila melanogaster.

All Circadian rhythms are regulated by an internally located clock, "body clock," or biological clock which is also known as circadian pacemaker, circadian system, and circadian oscillator.
The exact nature of the clock is unknown but the clockwork mechanism is undoubtedly physiological and involves both neuronal and hormonal control.
It tells our bodies when to sleep, rise, eat etc.,
The biological clock regulates sleeping and feeding patterns, alertness, core body temperature, brain wave activity, hormone production, and regulation of glucose and insulin levels, urine production, cell regeneration, and many other biological activities.
Though circadian rhythms are endogenously generated, they can be controlled by external signals such as sunlight and temperature. The length of day or night is important to both plants and animals.

The biological clock in mammals is located in the suprachiasmatic nucleus (SCN) of the hypothalamus of the brain. It is formed by 2000 neurons.
The information regarding the lengths of the day and night will be received and interpreted by the SCN of hypothalamus and it will be passed on to the pineal gland, a tiny, pine cone shaped structure located on the epithalamus of the brain. In response to it, the pineal gland secretes the hormone melatonin and its production will be maximum at night and minimum during the day.

The circadian rhythm tends to coincide with the cycle of daytime and night-time. During night, when it is dark, the eyes send a signal to the hypothalamus informing that it’s time to feel tired. In turn, the brain sends a signal to the pineal gland to release melatonin hormone, which causes drowsy feeling.
Melatonin provides information about length of the night. The onset of secretion of melatonin appears about two hours before natural sleep time and it reaches its maximum level during the middle of the night.

All biological clocks are controlled by certain genes. When the genes are activated, protein synthesis will be initiated and the proteins are produced. When the protein levels become maximum, the genes become inactivated and the protein synthesis will stop. When the protein levels drop over time to a point, it causes the genes to be activated and causes the biological cycle to start again.
In this way the biological clock is generated   and this process takes about 24 hours.
Light resets the biological clock and depending on the timing, light can advance or delay the circadian rhythm. Sleeping at night and waking up during the day is an example of a light-related circadian rhythm.

The concept of jet lag is an example of it, where the human internal physiological circadian rhythm is out of step with the Day and night rhythm of the destination.
In the case of jet lag, the travel disrupts the circadian rhythm. When a person travels through different time zones, the biological clock will be different from the local time.
For example, when a person travels from the west coast of California to the east coast of New York, he may lose 3 hours. When he wakes up at 7:00 a.m. on the east coast, his biological clocks are still running on west coast time, so he feels the same way as what he might be at 4:00 a.m.  Resetting of his biological clocks may often take a few days.

Caught my friend sleeping while waiting for boarding.
Photo by Joyce Romero / Unsplash

Airline pilots are unable to maintain sleep patterns that correspond to the natural human circadian rhythm because they have to spend many hours awake both day and night while crossing several time zones and regions of sunlight and darkness in one day. Such conditions can easily lead to fatigue.

People suffering from “delayed sleep phase disorder” cannot sleep at a normal time at night. They may stay awake until 2 a.m. or beyond. Elder people commonly suffer from “advanced sleep phase disorder”. In this, they feel very sleepy in the early afternoon and go to bed earlier than normal. As a result, they wake up too early in the morning and cannot go back to sleep. In the case of people suffering from an irregular sleep-wake rhythm, they cannot set a sleep pattern.

Distraction of the circadian clock causes the development of metabolic disorders which show adverse effects on health.  It may cause sleep disorders, obesity, mental health disorders, jet lag, and other health problems.
Shift-work leads to increased metabolic risks for cardio-metabolic syndrome, hypertension, altered insulin sensitivity, inflammation and higher body mass.


A. Sandhya

M.Sc Zoology

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