Types of biorhythm

Circadian

Variation is over an approximate 24 hour period.  The word stems from the Latin; circa (meaning ‘about’) and diem (meaning ‘day’).  There are some cycles that we are consciously aware of; the sleep/wake cycle being an obvious one, but most cycles we are not usually aware of.  For example our core body temperature fluctuates over a 24 hour period.  Generally it peaks mid afternoon at about 37.1 C and troughs in the wee small hours at about 36.7 C.  This may not sound like a lot but you may nevertheless have noticed the effect and found yourself shivering unexpectedly as you’ve walked home after a late night party, even in August!

Other examples of human circadian rhythm include heart rate, metabolism and breathing. These follow a similar pattern to temperature, which may not seem surprising, since they match our patterns of activity.  However, people on shifts, who are sleeping through the day and more active at night still keep the same circadian rhythms with body temperature, metabolism and resting heart rate still peaking mid afternoon!

Blood clotting also shows a circadian rhythm, peaking in the morning and coinciding with increased incidence of heart attack

It is worth mentioning that there are big differences between individuals.  The most noticeable being the larks/owls division; larks being morning types and owls preferring the evenings.  Typically when studied larks seem to be clock advanced having rhythms about two hours ahead of owls.

Research into circadian rhythms

For a detailed overview of research into these rhythms, seel the later notes on endogenous and exogenous control of rhythms, for example the work of Siffre and Morgan.

Endogenous

There is plenty of evidence to suggest that our biological rhythms are inherited.  For example although within a species there is variation of rhythm, each individual tends to have a pattern of rhythm that shows little variation over a lifetime.  Even the most extreme of environmental factors such as anaesthesia (not the late Russian Princess), alcohol and drug abuse, brain damage and loss of consciousness have little effect on our rhythms. 

To study endogenous clocks it is necessary to isolate people from external cues for many months.  In 1962 Aschoff and Wever studied a number of volunteers that agrred to spend time cut of from the outside world in a disused WWII bunker in Munich.  After a month or so cut off from external ues they adopted a 25 hour daily cycle. 

Above we see Siffre wired for various physiological readings during his 1972 isolation in Midnight Cave, Texas

However, French geologist Michael Siffre is the daddy!  Over the past 40 years or so he has regularly spent extended periods of time in various caves around the World and agreed to be studied during the process.  His first stint was in 1962 when he spent 61 days in a cave in the Alps.  He emerged on September 17^th but thought it was August 20^th!  (Note: if you add this up it simply doesn’t make sense.  This means he had lost 28 days which is more than 1 hour a day!).  In 1972 he was monitored by NASA in the caves of Texas and in 1999 he missed the millennium celebrations in a cave in some part of the World.  Each time his body clock extended form the usual 24 to around 24.5 hours. 

This appears to suggest two things:

1. There is internal control of the circadian rhythm, since even in the absence of external cues we are able to maintain a regular daily cycle.
2. There must usually be some external cue that keeps this cycle to 24 hours.  When this is removed we adopt this very strange 24.5 or 25 hour cycle. 

Evaluation

Some have criticised this research since it is so artificial.  In particular they object to the use of strong artificial light by the participants.  On waking the volunteers such as Siffre switch on lights which are likely to artificially re-set the body clock.  Czeisler et al (1999) has argued this is the equivalent of providing powerful drugs. 

In their own version, Czeisler et al kept 24 participants in constant artificial low-level light for one month and put them on a 28 hour cycle.  When readings of body temperature and blood chemicals were analysed they were shown to have adopted a cycle of 24 hours and 11 minutes, much closer to the 24 hours we would expect. 

Others however, disagree with Czeisler.  In an attempt to find the endogenous clocks’ period volunteers have been exposed to severe variations in clock alteration, for example, exposing participants to artificial lighting simulating a 28 hour day.  (So if ‘sunrise’ was at 6am on day 1 it would be at 10am on day 2 and so on).  The body cannot adjust to such extremes and the body clocks ‘run free.’  In all cases the cycle is greater than the usual 24 hours but estimates vary as to the exact length.  Some put the increase at as little as 11 minutes whereas others claim one hour.

The biological rhythm often comes into conflict with external cues, however, although disruption may occur the cycle remains intact.  If you have ever stayed up all night at a party and walked home in the early hours you may have felt cold, perhaps getting the shivers, even though it was August.  This is your natural dip in body temperature still occurring despite the night’s activities.

Evaluation of endogenous factors

Its role in the breeding cycle of humans is not so clear cut.  We do know however, that light levels can influence the menstrual cycle.  The menarche (onset of menstruation) is more likely to occur in the winter months and generally occurs earlier in girls that are blind.  In Finland, during its very long summertime daylight hours, conception rates increase significantly.  Perhaps you can think of other contributory factors!  No football for example

However, much of the research has been carried out on non-human animal species so we have issues of generalisation.  Breeding behaviour in humans is far more complex.  The females of many species, including rats, simply need to be exposed to the right pheromone for them to adopt the lordotic stance (legs akimbo, bottom raised mating position).  Clearly, outside of Essex at least, this is not the case with human females. 

When humans participants have been involved they have tended to be case studies of individuals, and very often individuals with medical conditions, so again, generalisation is an issue. 

Biological basis of circadian rhythms

In lower species the pineal gland appears to be the brain structure responsible for regulating bodily rhythms, especially the sleep/wake cycle.  The pineal gland lies at the top of the brainstem and in lower species this means it is close to the surface of the skull.  As well as having an inbuilt cycle it also has light sensitive cells that receive information through the skull about external light levels and these seem to keep it synchronised with fluctuating environmental conditions.  The pineal gland secretes melatonin which is known to have an influence on sleep patterns.

In humans the pineal gland is still situated in the same place, at the top of the brainstem, but we have an extensive cerebral cortex overlying this.  (When I say ‘we’ I refer to most of us!).  This means the pineal gland is situated deep inside the brain so has no direct contact with conditions outside.  (In fact the Greeks considered the pineal gland to be a possible site for the ‘soul’ since it was situated in the centre of the brain).

In humans the suprachiasmatic nuclei (SCN) appears to take over the role.  This is situated in the hypothalamus and just behind the eyes and receives sensory input about light levels through the optic nerve.  The SCN then appears to regulate melatonin production from the pineal gland.  Removal of the SCN in rats causes the usual sleep/wake cycle to disappear. Studies of the electrical activity of the SCN show it to have a cyclic activity varying over a 24.5 hour cycle.  This cycle persisted even after the SCN had been removed from the brain (so called ‘hypothalamic island’). 

Humans certainly have a bunch of fibres (the retino-hypothalamic tract) that connects, as the name suggests, the retina of the eye to the hypothalamus.  As early as 1929 Bailey found that patients with brain tumours close to the hypothalamus suffered from odd sleep/wake cycles. 

Evidence suggests that there is more than one internal clock.  Damage to the hypothalamus of squirrel monkeys alters their sleep/wake cycle but leaves other cycles such as metabolism and temperature unaltered, (Fuller et al 1981).

Body clocks everywhere

The SCN appears to be the location of the main clock but there are certainly others.  Yamazaki et al (2000) found that tissues from the liver, lungs and other organs could maintain a constant 24 hour cycle despite being kept in vitro (outside the body). 

Additional biological content for extension purposes

The SCN may regulate other cycles.  Rusak & Morin (1976) found that lesions to the SCN disrupted their breeding pattern (infradian rhythm).  Instead of just producing testosterone during the mating season, the hamsters produced it all year round.  Morgan (1995) removed the SCN from some hamsters and found that their rhythms ceased.  However, when they received SCN transplants from other hamsters the cycles were re-established.  In a follow up they transplanted the SCN from mutant hamsters (15 feet long!) who had shorter circadian rhythms.  The hamsters receiving these SCNs developed these mutant cycles.  (However, they did not grow to the same length!).

Mutant hamsters have been used as a green source of electrical energy in London.  Using the London Eye as a giant wheel they can generate enough electricity to power Westminster and half of Kensington!

External Factors (zeitgebers)

Light appears to be crucial in maintaining 24 hour cycles: 

Campbell & Murphy (1998), in a bizarre experiment, shone bright lights onto the back of participants’ knees and were able to alter their circadian rhythms in line with the light exposure.  The exact mechanism for this is unclear, but it seems possible that the blood chemistry was altered and this was detected by the SCN. 

Miles et al (1977) reported the case study of a blind man who had a daily rhythm of 24.9 hours.  Other zeitgebers such as clocks, radio etc. failed to reset the endogenous clock and the man relied on stimulants and sedatives to maintain a 24 hour sleep/wake cycle.

Luce & Segal (1966), however, have shown that light levels can be over ridden.  In the Arctic Circle people still maintain a reasonably constant sleep pattern, averaging 7 hours a night, despite 6 months of darkness in the winter months, followed by six months of light in the summer.  In these conditions it appears to be social factors that act to reset endogenous rhythms rather than light levels. 

Conclusion

Our biological rhythms therefore appear to be internally and externally controlled.  Left to their own devices our internal clocks seem to be set to about a 25 hour cycle but external cues, especially light, resets our clocks daily.  So why do we need internal and external control?  If control was entirely internal we would not be sensitive to external changes such as light levels.  Species that hibernate or migrate would not adjust their behaviour.  If control was entirely external our rhythms would be too erratic and change day to day depending on weather conditions etc.

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