Wednesday, June 9, 2010


It seems natural but why in a 24 hours day do we sleep at night and work in day, and why does ‘shit happens’ mostly in the mornings? Our biological routines are mostly time bound…and that too round the clock.
A clock can be defined as an instrument used to indicate, keep, and co-ordinate time. Clocks have advanced from being the sun dials of ancient times to the ultra modern atomic clocks.

But there is another clock, besides the one hanging on the wall… inside our body. It is fitted in our brain. It determines the rhythmicity of our biological routines like being alert in day, sleepy at night. And the study of “biological time” or biological rhythmicity is called Chronobiology.

Among biological rhythms, circadian (in Latin: circa= about; dies= day) rhythms are the most extensively studied. The activity–rest (i.e. wake-sleep) cycle is the most apparent of all circadian rhythms.
There are various less overt phenomena going on inside our body which depend on this clock, such as many hormones are released in a pulasatile manner, heart attacks are more common in early mornings, the blood pressure varies at different times of day- being maximum in evenings, the normal body temperature is maximum in the evenings and lowest in early morning before dawn. The fever of tuberculosis also has an evening rise; the alertness in daytime also varies in between the day, being most alert in the forenoon. Although there can be exceptions… these processes are more or less time bound.

This human ‘clock’ resides in an area of brain- the anterior hypothalamus and is called suprachiasmatic nucleus (SCN) by the scholars. And it does not agree with the wall clock.

The mean circadian period generated by the human clock (or SCN) is approximately 24.18 hours. The wall clock completes an astronomical day/cycle in 24 hrs. So the human watch ticks 10 minutes and 48 seconds too slowly per day. Therefore, a circadian/human clock must be reset on a regular basis to be effective at maintaining the proper phase relationships of behavioral and physiological processes within the context of the 24-hour day. If not done, an individual will gradually come out of synchrony with the astronomical day. In slightly more than 3 months, a normally diurnal human would be in opposite phase to the day–night cycle and thus would become transiently nocturnal i.e. wakeful at night and sleeping in day.
Best wishes for your Time Check.


  1. is that really true boss?? i never knew d human circadian rhythm has been calculated that precisely to the second ... doesnt sound that credible, does it, especially when considering the vast individual variations??

  2. yes anurag, that is true, as per current evidence. It is calculated as an average, and may differ in individuals.
    For reference u can check out
    1- 'Kaplan & Sadock's Comprehensive Textbook of Psychiatry' 9th edition
    2-Czeisler CA, Duffy JF, Shanahan TL, et al: Stability, precision, and near-24-hour period of the human circadian pacemaker. Science. 1999;284:2177.
    3-Miller JD, Morin LP, Schwartz WJ, Moore RY: New insights into the mammalian circadian clock. Sleep. 1996;19:641.
    4-Reppert SM, Weaver DR: Coordination of circadian timing in mammals. Nature. 2002;418:935.
    ....if u want more gruesome details.

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