by Jacqueline Nicole Aw 歐婷梅
Circadian rhythm, or circadian clock, is an internal biological clock that regulates sleeping and feeding patterns, as well as a whole host of other biological functions in all living organisms. Organisms most active in the daytime are diurnal, humans included, whereas nocturnal organisms come alive at night. With heightened senses most appropriate for nighttime, nocturnal organisms enjoy a multitude of advantages – most importantly a reduction in the direct competition for food. What biological factors administer to the internal clocks and what are the differences between diurnal and nocturnal circadian rhythms?
Nocturnal organisms typically possess enhanced eyesight, hearing and smell to compensate for the lack of light. Saucer-like eyes that allow maximum light are distinctive in characteristic nocturnal organisms such as owls and loris. Bats on the other hand, emit constant streams of high-pitched noises that bounce off objects to assist them in navigation in the dark. Meanwhile, many nocturnal organisms spend the daytime catching up on some shuteye or grooming. The activity levels of both diurnal and nocturnal organisms are governed by the circadian rhythm.
A self-sustaining oscillation, the circadian cycle in living organisms has a period of roughly over 24 hours. In addition to regulating sleep, it also affects the physiology, endocrine system and behaviour of organisms. Together, organisms make use of the circadian rhythm to derive maximum benefit from temporarily available resources. For instance, they are able to anticipate daily food availability and pressure from predators ahead of time; cycles are typically not 24 hours exact. External timing cues known as zeitgebers assist in synchronising the cycle with geophysical time by “resetting” or changing the phase of the circadian clock. The most powerful zeitgeber is light stimulation . In addition, scheduled voluntary exercise or food shortages are also capable of shifting the phase.
Although nocturnal and diurnal organisms exhibit an almost antiphase in physiology, metabolism and behaviour; the main characteristics of their circadian metabolisms are largely parallel. In both cases, the master clock in the circadian cycle is the suprachiasmatic nucleus (SCN), located in the hypothalamus . The SCN receives light stimuli and “resets” the cycle in order to be synchronised with the environment. The hormone responsible for the regulation of the circadian rhythm is called melatonin, and its synthesis and release is controlled by the light-dark cycle. This hormone then regulates other biological processes to reset the clock depending on whether the animal is nocturnal or diurnal.
SCN is also responsible for the release of a hormone known as vasopressin. The main target of vasopressin is the neuroendocrine system (the hypothalamus-pituitary-adrenal (HPA) axis), which regulates numerous biological processes, such as digestion, immune system and stress levels. Recent studies have shown that most activity from the HPA axis happens when light diminishes in nocturnal organisms, whereas the opposite is true for diurnal organisms . This allows the organism to adjust to the demands of the day’s actions.
Along with features made optimal for night vision and sensitive hearing, nocturnal organisms are well-adapted to lightless environments with the help of nature’s custom designed biological clocks. Intricate and complex systems in both nocturnal and diurnal animals, circadian rhythms are regulated by a whole host of factors and can influence sleep-wake cycles, body temperature and important biological functions. Recent research on this topic focuses a lot on identifying the genes which dictate how nocturnal and diurnal animals react to light stimuli, but much has yet to be done to fully decipher these intricate biological codes.
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