All biological clocks are adaptations to life on a rotating planet.
— Colin Pittendrigh, undated.
Internal timing mechanisms
Activities and behaviors
are a great way to
release etc.) as well
as ultimate (fitness,
etc.) questions (e.g.,
activities, timing of
General patterns of rhythms (these concepts and labels are
universally applicable to other phenomena such as sound)
Biological rhythms can be categorized
based on their frequency and periods.
Epicycles (ultradian, variable lengths of time):
ultradian rhythm is defined as a regular (usually
physiological) cycle or oscillation (e.g., of hormone levels)
that takes less than a day and sometimes a very short
period of time to complete.
This is from a
we all take 20
minutes to relax
I had that kind
Tidal Rhythms (12.4 hours):
The ebb and flow of tides
affects the behavior of some animals occupying the tidal
These occur on a daily basis, monthly bases, as
well as yearly cycles.
are related to tidal
rhythms, but can affect animals that are not necessarily in
the tidal zones.
Circadian rhythms (24 hours):
are probably the best known.
Diurnal: Active in daylight
Nocturnal: Active at night
Crepuscular: Active at dusk and/or dawn
Circadian rhythms can be altered over a yearly period.
Example: Bird species that live in northern temperate habitats
throughout the year can switch from crepuscular activity in the spring
and summer to diurnal activity in the winter.
Circadian rhythms can also be altered over the lifetime of an individual
woodchucks are active in
early evening, but adults
are more diurnal in their
A typical human circadian rhythm.
Circannual rhythms (12 months)
are behavioral and
physiological patterns that are governed by self-sustaining
internal pacemakers and that occur within a period of about 1
and diapause (a
Possible human circannual cycles:
(Seasonal Affective Disorder; SAD)
Diastolic blood pressure and heart rate
(Randall, 1991; greatest in summer)
T, FSH and LH:
In the controls, annual rhythms were
validated for the secretion of T (annual crest time in
fall and winter), LH (annual crest time in February), FSH
(annual crest time in January) and
(crest time September)
This data for humans in
North America strongly
patterns in reproduction.
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