Tuesday, 13 March 2012

The Human Body Clock, Circadian Rhythms and Chronobiology

The human body clock is perhaps not as simple as one might think. Does it run on day light alone as some suggest? Why do we find ourselves unable to sleep in the middle of the night sometimes as if our body clock has shifted? Why does jet lag affect us so monumentally? The following article was taken from http://thebrain.mcgill.ca/flash/a/a_11/a_11_p/a_11_p_hor/a_11_p_hor.html and sheds a lot of light on what scientists have found out about chronobiology...

The research field now known as chronobiology deals with the body’s biological rhythms: the way that it generates its physiological oscillations and keeps its various systems synchronized.

Ever since the first findings suggesting that cycles of varying length occur in the human body, scientists’ understanding of chronobiology has rapidly grown more complex. They quickly realized that daylight alternating with darkness was not the source of human circadian rhythms, but simply the means by which a truly endogenous central biological clock within the body is kept synchronized with the hours of day and night. This central clock that is synchronized by daylight is located in the suprachiasmatic nuclei of the brain.

Scientists also soon discovered the importance of this cyclicity in most of the body’s major systems. Whatever physiological variables researchers measured—such as cell metabolism, body temperature, or the secretion of various hormones—each seemed to fluctuate in a cycle with its own specific peaks and troughs.

Many experiments have been conducted to uncover the subtle connections among these various rhythms. Some of the greatest insights have been provided by temporal-isolation experiments, in which subjects are completely isolated from the usual cues of alternating day and night, such as daylight and traffic noises. Numerous researchers have conducted such experiments, to examine questions such as whether, under such conditions, people continue to fall asleep at their usual time, or whether their activity cycle instead begins to run too fast or too slow.

The first temporal-isolation experiments were conducted in caves, where the temperature is naturally constant and where subjects can be completely isolated from the outside world. The first major finding from these experiments was that the subjects’ circadian rhythms persisted despite this isolation, which proved that all human beings have an “endogenous clock” inside their brains.

But these experiments also showed that this clock was not perfectly accurate: it lost a few minutes every day. In other words, the subjects’ natural endogenous circadian cycle was slightly longer than 24 hours, ranging from 24.2 to 25.5, depending on the study. This may not seem like much, but if someone’s cycle lasted 24.5 hours instead of exactly 24, then within 3 weeks, everything that he used to do in the daytime he would end up doing at night!

These temporal-isolation experiments date back quite some time. As early as 1938, Nathaniel Kleitman and his colleague Bruce Richardson spent 32 days in a cave in the U.S. state of Kentucky, deprived of all time cues. In 1962, the French researcher Michel Siffre spent two months in an underground glacier in France’s Maritime Alps. He was 23 years old at the time of this first experiment, and he spent two more long periods underground later in his career to measure how the absence of time cues affected his biological rhythms at various ages. The third time, in 2000, he was 61 years old and stayed underground with no time cues for 73 days (see below).

One of the most spectacular observations during these time-isolation experiments in caves, laboratories, and other settings is the way that subjects’ sleep-wake cycles shift relative to the actual alternation of day and night in the outside world. But as soon as the experiments are over, the subjects take only a few days to resynchronize their cycles to these external time cues. 

Another interesting phenomenon in these experiments is that in some cases, the time of lowest daily body temperature shifts from the end of the sleep period to the start of it. Thus time isolation may produce shifts not only in behavioural cycles (such as sleeping and waking) but in physiological cycles (such as that for body temperature) as well. This desynchronization is the likely source of the problems associated with jet lag.

The Cave study: Michel Siffre (1972)

Aim: This study investigated what would happen to people’s circadian rhythms if they were cut off from all zeitgebers (signals from outside the body that tell us about the time of day – such as light and dark, clocks), and had to rely on their endogenous pacemaker (internal body clock) to tell them when to eat and sleep.  Would we still stick to a natural 24-hour rhythm?

Method:  Michel Siffre, a French cave explorer, spent over six months living in a cave in Texas, deep under the ground, with no light, or anything else to tell him what time of day it was.  His biological clock was allowed to ‘free-run’, that is, he just followed his body’s inclinations, eating and sleeping whenever he chose, with no fixed timetable.  He was wired up so that some of his body functions could be recorded; he had a telephone link to the outside world, and was monitored by video camera. 

Results: Siffre had a fairly erratic sleep-wake pattern at first, but it settled down to a pattern that averaged just over 25 hours, instead of 24 hours.

Conclusion: We do have an internal mechanism that regulates our sleep/wake cycle, but it shifts to a length of approximately 25 hours if we do not have external zeitgebers to reset it.

Evaluation: This is a one-participant study, so may not be generalisable to all humans.  Also Siffre’s living conditions were unusual in other ways than simply lacking time signals, and other factors such as loneliness could have affected his behaviour.  Similar studies have been done with rats, isolating them from daylight (Groblewski), and found a similar increase in the sleep-wake cycle, which supports the findings from the Siffre study.  A strength of the study is that it lasted a long time, allowing Siffre’s rhythms to settle down into a natural pattern.

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