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 d i u r na l and noctu r na l c i rcad i an
rhythms?
Noct u r na l o rgan i sms t yp i ca l l y pos ses s
enhanced eyes i ght, hea r i ng and sme l l 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
Nocturnal vs. Diurnal
Biogical Clocks
生物時鐘比較
By Jacqueline Nicole Aw
歐婷梅
References
[1] Refinetti, Roberto. “Comparison of the Body Temperature
Rhythms of Diurnal and Nocturnal Rodents.” The Journal of
Experimental Zoology (1996): 67-70.
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 [1]. In
addition, scheduled voluntary exercise or food
shortages are also capable of shifting the phase.
Although nocturnal and diurnal organisms
exh i bi t an a lmos t ant i phase i n phys i o l ogy,
me t a b o l i s m a n d b e h a v i o u r ; t h e ma i n
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
[2]. The SCN receives light stimuli and “resets”
the cycle in order to be synchronised with
the environment. The hormone responsible
