Circadian Rhythm and the Human Biological Clock

A circadian rhythm, also known as a circadian cycle (biological rhythm), is a normal, internal mechanism that controls the sleep–wake cycle and occurs each 24 hours or so. It may apply towards any process which originates (is endogenous) inside an organism and reacts to its surroundings. A circadian clock is responsible for these 24-hour rhythms, which have been found in fungi, animals, plants, and cyanobacteria.

Circadian is derived from the Latin words circa, which means "around" (or "roughly"), and dim, which means "day." Diurnal rhythms involve processes of 24-hour cycles; circadian rhythms ought not be considered diurnal rhythms until they can be established as endogenous and therefore not environmental.

While circadian rhythms are endogenous, external cues named zeitgebers (German meaning "time givers"), such as temperature, light, and redox cycles, adapt them to the local setting. A circadian rhythm sleep disorder is a term used in clinical settings to describe an irregular circadian rhythm throughout humans.

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Criteria

A Biological Rhythm Must Fulfil Three General Requirements to be Considered Circadian:

1. There is an endogenous free-running cycle in the rhythm which lasts about 24 hours. The rhythm lasts for approximately 24 hours under constant conditions (— for example, constant darkness). The free-running era, signified through the Greek letter (tau), is the duration of the rhythm in constant conditions. Such criterion was created to differentiate circadian rhythms from basic feedback to regular external cues. It is impossible to call a rhythm endogenous until it has been checked and continues in the absence of an external periodic input. In diurnal animals (those that are active throughout daylight hours), is significantly longer than 24 hours, while in nocturnal animals (those that are active at night), is noticeably smaller.

2. The rhythms are hypnotic. Entrainment is the method of resetting the rhythm through exposing it to external stimuli (including light and heat). The Zeitgeber, or "time giver," seems to be the external stimulus that is being used to entrain a rhythm. Traveling through time zones demonstrates the human biological clock's ability to adapt to local time; an individual would normally experience jet lag until their circadian clock has been synchronised with local time.

3. Temperature compensation is evident in the rhythms. They preserve circadian periodicity over a wide range of physiological temperatures, in other words. Numerous organisms reside in a wide range of temperatures, and thermal energy variations can influence the kinetics among all molecular processes within their cells (s). The circadian clock of an organism would retain an approximately 24-hour periodicity amid shifting kinetics in order to ensure continuity of time, a property referred to as temperature compensation. Such compensating effect is measured by the Q10 temperature coefficient. The rhythm is said to have been temperature-compensated unless the Q10 coefficient stays about 1 as the temperature rises.

Importance in Animals

Animals, like humans, have circadian rhythmicity in their feeding sleeping habits. Core body temperature, cell regeneration, hormone development, melatonin secretion, brain wave activity, as well as all other biological activities, Travelling have distinct patterns. Furthermore, photoperiodism, or maybe an organism's physiological response to the duration of day or night, is critical for both animals and plants, and the circadian system is involved in day duration calculation and perception. Numerous species rely on accurate forecasting of seasonal weather patterns, food availability, and predator behaviour to ensure their survival.

Effect of Circadian Disruption: The role of body clocks in ensuring the correct pacing of cellular/metabolic events has been illustrated in mice through deletions or mutations of the clock gene; clock-mutant mice become hyperphagic and obese, with altered glucose metabolism.

In mice, deletion of the Rev-ErbA alpha clock gene promotes obesity and alters the equilibrium between lipid and glucose utilisation, putting them at risk for diabetes. However, this is unclear if there is a direct link between human clock gene polymorphisms as well as the risk of developing metabolic syndrome.

Effect of Light–dark Cycle: The light–dark cycle is related to the rhythm. Animals, like humans, who are held in complete darkness for long periods of time develop a free-running rhythm. Based on whether the "day," or endogenous time, is longer or shorter than 24 hours, the sleep cycle has been pushed back and forward each "day."

Arctic Animals: Researchers from the University of Tromso in Norway discovered that certain Arctic species have circadian cycles during the months when there are frequent sunrises and sunsets. Reindeer at 70 degrees north exhibited circadian cycles in the winter, autumn, and spring, though not in the summer, according to a report. Just in the autumn and spring did reindeer on Svalbard, at 78 degrees North, exhibit these rhythms.

Butterfly and Moth: The Eastern North American monarch butterfly (Danaus plexippus) utilizes a time-compensated sun compass which relies on a circadian clock throughout the antennae to navigate the fall migration to their overwintering grounds throughout central Mexico. Females of some of the moth species, including Spodoptera littoralis, emit a particular pheromone which attracts as well as resets the male circadian rhythm to trigger breeding at night, and this is believed to regulate mating behaviour.

Circadian Rhythm in Plants

• Plant circadian rhythms notify each plant what season it is and when it is supposed to bloom to maximise pollinator attraction. Leaf movement, germination, stomatal/gas exchange, development, photosynthetic activity, fragrance emission and enzyme activity are just a few of the behaviours that display rhythms. 

• Circadian rhythms develop like a plant synchronises its light cycle with that of its surroundings. Such rhythms seem to be self-sustaining and endogenously produced, and they stay largely stable across a wide range of ambient temperatures. 

• Two interacting transcription-translation feedback mechanisms, proteins with PAS domains, that promote protein-protein interactions, and many photoreceptors which fine-tune the timer to distinct light conditions, are crucial elements. 

• Anticipating extreme weather events enables plants to make necessary physiological changes, giving them an adaptive advantage.

• In agriculture, a greater understanding of plant circadian rhythms could aid farmers stagger crop harvests to prolong crop availability and protect against weather-related losses.

Circadian Rhythm in Humans

• Biological clock of human body: When separated from external factors including daylight and timekeeping, early studies into circadian patterns indicated that almost all people favoured a day nearer to 25 hours. 

• Nevertheless, this study was flawed since the researchers were not shielded from artificial light. The scientists were unaware of the phase-delaying impact of indoor electric lights because the subjects weren't really exposed to time signals (such as clocks) or daylight. 

• Whenever the subjects stayed up, they could switch on the sun, and then when they tried to sleep, they could switch it off. The circadian process of biological clock in humans was delayed by exposure to electric light throughout the evening. According to a more rigorous study published by Harvard University in 1999, the natural human rhythm is similar to 24 hours and 11 minutes, which is also nearer to the solar day. 

• A much more new analysis from 2010 found sex differences within circadian cycle for men and women, with women's periods being significantly shorter (24.09 hours) than men's (24.19 hours). 

• Females in this study appeared to be awake faster than men and preferred morning tasks more than men, despite the fact that the biological mechanisms underlying such variations remain unclear.

Facts on Circadian Rhythm

• Circadian rhythms are 24-hour cycles of physical, emotional, and behavioural changes that are mainly triggered by light and darkness in an organism's environment.

• Our circadian rhythms are regulated by our biological clocks. Internal clocks are collections of interacting molecules found in cells all over the body. All of the body clocks are in sync thanks to a "master clock" in the brain.