Biological rhythms are a great way to study proximate (mechanistic) questions (nervous system, sensory systems, hormonal release etc.) as well as ultimate (fitness, etc.) questions (e.g., peak feeding activities, timing of reproduction).
General patterns of rhythms (these concepts and labels are universally applicable to other phenomena such as sound) Phase shift Period Amplitude
Biological rhythms can be categorized based on their frequency and periods. Epicycles (ultradian, variable lengths of time): An 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 book and website suggesting that we all take 20 minutes to relax every 90 minutes. (Wish I had that kind of time!)
Tidal Rhythms (12.4 hours): The ebb and flow of tides affects the behavior of some animals occupying the tidal zone. These occur on a daily basis, monthly bases, as well as yearly cycles.
Lunar rhythms (28-day cycle): 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 animal. Example: Young woodchucks are active in early evening, but adults are more diurnal in their activity patterns.
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 year. Examples: Hibernation (some vertebrates) and diapause (a period of dormancy in insects, some fish)
Possible human circannual cycles: Mood (Seasonal Affective Disorder; SAD) Diastolic blood pressure and heart rate (?) Hair growth (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 late fall and winter), LH (annual crest time in February), FSH (annual crest time in January) and sexual activity (crest time September)
This data for humans in North America strongly suggests circannual patterns in reproduction.
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