Biological clocks are not mechanical. A biological clockwork results from complicated interactions between several genes and their protein gene products.
Copyright 1996 Corel Corporation.
Have you ever experienced extreme exhaustion after traveling
long distances? Or wondered how animals knew when it was time to hibernate?
Have you ever seen plants capable of raising and lowering their leaves at
certain times during the day?
These phenomena can be at least partially explained by internal timing
mechanisms known as biological clocks. Biological clocks give organisms a rhythmic pattern to follow daily, monthly or even
yearly. In 1729, a Frenchman named M. de Mairam found that those plants
that are noted for raising and lowering their leaves rhythmically throughout
the day maintain this rhythm even when kept in constant darkness. That is,
they maintain this pattern even in the absence of environmental cues, such
as light and temperature. Such a rhythm is termed endogenous. Rhythms
that stop without environmental "hints" are termed exogenous. The actual environmental cue that is capable of setting the phase of a biological
rhythm is a phasing factor or Zeitgeber (German for "time-giver"). Often, in the absence of environmental cues, an endogenous rhythm continues in periods close to, but not exactly,
24 hours. Such rhythms are circadian rhythms, meaning
the rhythm continues without exogenous cues, but is only "about"
a day in length. In the normal situation, circadian rhythms are corrected
by a Zeitgeber (usually daylight) resulting in rhythms with a period of
almost exactly 24 hours.
Why are these internal clocks important? Why cannot an animal depend
only on environmental cues? One advantage is that biological clocks allow
animals to anticipate and prepare for upcoming events. Imagine an animal
that is active at night. If it is solely dependent on external cues, it
does not know how much time is left until dawn. Clocks also permit a relatively
accurate timing of the periods during the day, even when environmental cues
are vague, difficult to use or less reliable (like temperature, which can
fluctuate). Circadian clocks reacting to changes in day length may be used
by hibernating animals to determine the time of year, although conclusive evidence for this is lacking. A hibernating animal
must have an internal clock reminding her that winter is approaching. Otherwise she might not store enough fat in time for the winter, simply waiting
for the weather to get cold. Also, animals depending on daily available
light might utilize internal clocks that detect changes in the length of
daylight as a sign for reproduction, migration or moulting. Some examples
of other processes showing circadian rhythmicity include locomotor activity
in many vertebrates and insects, variations of body temperature in birds
and mammals, and colour changes in fish and crabs.
What about jet lag? Jet lag results when a person suddenly experiences
a shift of many hours in the phase of the environmental cycle, after travelling
long distances over a relatively short period of time. The person suffers
from a disturbance of the normal relation between her internal biological
clocks and the external environmental cycle. Some of the internal rhythms
lag behind others in attempting to regain synchrony with the cycle of the
outside world. Thus, the person experiences the symptoms of jet lag as her body tries to
compensate for the differences between all her internal rhythms and the
Thus, biological clocks serve as automatic internal "watches"
for organisms, even in the absence of obvious environmental cues. Biological clocks are very probably found in all organisms, from bacteria to people.
The text has been updated and enlarged in 2013 by Dr Anders Lundquist (senior lecturer at the Department of Biology, Lund University, Sweden).
R.W. Hill, G.A. Wyse, and M. Anderson: Animal Physiology (3rd ed, Sinauer, 2012).
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