Biological rhythms

Biological rhythms are an integral part of everyday life for most organisms on earth. They regulate most important functions in each organism. In plants circadian clocks control flowering, in response to seasons and photosynthesis. In mammals, circadian clocks manage sleeping, waking, feeding and controlling whether an animal is nocturnal or diurnal.

Behavior rhythms-definition

A rhythm has been defined as a sequence of events that repeat themselves through time in the same order and the same interval.

Simply a rhythm is a periodically recurring event. Many behavioral patterns of organisms are subject to rhythmic variations in response to various external geophysical rhythms  in the environment.

Biological rhythms- definition

Biological rhythms are those rhythms that arisen in organisms to match external geophysical rhythms with comparable period (Chandra sekaran 1986). A biorhythm means a periodic occurrence of specific physiological changes in living organisms. The majority of organisms show daily and annual cycles of activity and development.

Parameters of biological rhythms

Each biological rhythm is composed of repeating units called cycles.

The length of time required to complete an entire cycle is the period.

The magnitude of the change in activity rate during a cycle-the difference between peaks and troughs is the amplitude. Any specified recognizable part of a cycle is called a phase.

Properties of biological rhythms

·       Biorhythms have self – sustaining pacemaker mechanism.

·       Biorhythms maintain their normal cyclicity even in the absence environmental cues.

·       Biorhythms are unaffected by metabolic poisons or inhibitors.

·       Biorhythms are genetically transmitted.

General criteria of biological rhythms

The rhythms repeat in a given time period  for e.g. circadian rhythms in every 24 hours. The rhythms persist in the absence of external cues. The rhythms can be adjusted to match the local time ( entrain able ). The rhythms maintain circadian periodicity over a range of physiological temperatures.

Examples of endogenous rhythms – circadian(daily); circa tidal (sea –tide);circulunar (moon phase); circannual (year wise).

Zeitgebers=time givers = synchronizers

The mechanism whereby the period of a rhythm occurs repetitively and coincides approximately with the presence of some external stimulus is called entrainment. Cues that provide information to animals about periodicity of environmental variables are Zeitgebers.

Zeitgebers are the entraining agents defined as those cyclic environmental cues that can entrain free running endogenous pace makers. Zeitgebers can influence rhythms by effecting both the phase and the frequency.

Examples of Zeitebers

Daily light and dark cycles for terrestrial organisms.

Ebb and flow tides for intertidal marine organisms.

Temperature or light for ectotherms e.g lizards, insects.

Cycles of food availability,social cues for small mammals.

Classification of biorhythms

•         Physical classification-(period of oscillation) - Circadian-24h; Ultradian-less than 20h; Infradian- more than 28h.

•         Functional classification-(periodic physiological factors)- Alpha rhythm; Beta rhythm; Gamma rhythm.

•         Descriptive classification - Diurnal, Nocturnal, Serotine, Vesperal; Morning, Daily, Weekly, Monthly, yearly.

•         Endogenous rhythms- Circadian rhythms, tidal rhythms, Lunar rhythms, semi-lunar rhythms, Circannual rhythms.

•         Exogenous rhythms- Light intensity rhythms, temperature rhythms, pressure rhythms, geophysical rhythms

•         Short term  rhythms - Circadian rhythms- Feeding ,activity, hunger and sleeping rhythms.

•         Long term  rhythms- Lunar, semi-lunar,  tidal, circannual, life cycle, breeding, migration rhythms

Circadian rhythms - A rhythm with  a periodicity of 24 hours is called a circadian( from Latin circa ‘about’; di means day i.e. about a day) . E.g. sleep-wake cycle. There may be over 100 physiological cycles in human body. Circadian rhythms are endogenous. Circadian rhythms present even in the absence of environmental cues such as light, temperature or social cues.

Properties of circadian rhythms –

•         They are genetic in origin.

•         They are controlled by biological clocks.

•         The biological clocks are reset and calibrated by periodic environmental signals ( called synchronizers or Zeitgebers or entraining agents).

•         There are often cycles within cycles, their intensity and frequency varying in different parts of the activity period.

Human rhythms - Human body appears to be a clock – shop. There are several rhythms – one clock basis. Most of the rhythms are interdependent and coupled. There may be one master clock orchestrating all individual biological clocks. Certain biorhythms are the natural monthly fluctuations that govern physical, emotional and intellectual aspects of human beings.

•         Human beings have three biorhythms:

·       Physical biorhythm -23 days.

·       Emotional biorhythm – 28 days

·       Intellectual biorhythm – 33days

The circadian pace maker or master clock

•         The master clock controls circadian rhythms.

•         It consists of a group of brain cells in the hypothalamus called supra chiasmatic nucleus (SCN).

•         The hypothalamus continually measures the light exposures via the retinal hypothalamic tract and accordingly adjusts the timing of the sleep-wake cycle.

Endogenous rhythm theory - This concept suggests the biochemical nature of clock mechanism. The  internal biological clock operate by cyclic biochemical reactions. The biochemical endogenous clocks are sensitive to environmental cues.

Exogenous rhythm theory - According to this theory, intracellular rhythm occurs in response to cyclic geophysical changes.

Brown (1970) conceptualized the system has a two-layered ring.

There is a core of unchanging geophysical cycles existing under a changeable system of factors like light, temperature and food.

Rhythm disorders –

•         Seasonal affective disorder – SAD-is an infradian rhythm disorder. SAD may have disturbance in the melatonin system. Sad people show low mood effects in winter months.

•         Jet lag- influences our endogenous pace maker. Symptoms include tiredness, sleepiness, loss of concentration, anxiety, depression and irritability. Some people recover quickly but others the symptoms persist for days.

Chronotherapy refers to the use of circadian or other rhythmic cycles in the application of therapy. Chronotherapy is used in the treatment of sleep disorders, asthma, cancer, hypertension, and multiple types of depression, seasonal affective disorder and bipolar disorder.