There are three main biological rhythms, which are called infradian, circadian and ultradian. Ultradian rhythms occur multiple times in one day. Circadian rhythms are cycles, which occur once a day, so within the 24 hours such as the sleep-wake cycle. Infradian rhythms are less frequent once in a day such as the menstrual cycle or hibernation. The research into these rhythms offers an insight into how the body is influenced by exogenous zeitgebers and endogenous pacemakers, but it is still nonetheless unclear what it more influential.
An example of circadian rhythms is the sleep-wake cycle that evidence suggests that exogenous pacemakers control it. For example, the theory is that the suprachiasmatic nucleus has an involvement in controlling the sleep-wake cycle. Ralph et al investigated how the suprachiasmatic nucleus (SCN) may be involved in the sleep-wake cycle.
By examining hamsters that had a 20-hour sleep-wake cycle, he was able to extract the SCN and enter it into other hamsters with a 24-hour cycle. Ralph found that the group of hamsters with the 24-hour sleep-wake cycle eventually had a 20-hour sleep-wake cycle. Ralph’s study would suggest that endogenous pacemakers rather than other external influences control the sleep-wake cycle. However, Ralph’s experiment is very limited by the inability to be able to generalise this study to humans as this experiment is only done on animals. Yet Siffre came to similar conclusions as he took part in his longitudinal study.
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He found after taking himself out of humanity and into a cave for six months with only artificial light, taking away external cues, he found that his own body clock changed to 25 hours rather than a 24-hour sleep wake cycle. Therefore, with the external cues removed, this would suggest that endogenous pacemakers take an influential role in there.
Likewise, there is a problem with generalising this study to the general population as it is does on one person so it is a case study which affects both the reliability and the validity of the conclusions drawn.
The level of the hormone called melatonin in ones system affects the sleep-wake cycle. The more melatonin means there is less serotonin in the bloodstream and with more melatonin encourages the person to feel more tired and hence will make the person sleep. The melatonin levels are regulated by the activity of the SCN (an endogenous pacemaker) and by the levels of light that is detected (exogenous zeitgebers).
The SCN will react to the level of light that is being detected whereby the paracrine system begins releasing melatonin from the pineal gland.
Schochat et al. (1997) investigated the involvement of melatonin in the sleep-wake cycle. By having six males for 29 hours in a darkened room, he was able to wake them up every 20 minutes and take a blood sample and then measure the levels of melatonin against how quickly they feel asleep. The results demonstrated a close and precise link between the amounts they slept in those 20 minutes and melatonin levels, the higher the melatonin levels, the greater the propensity to sleep.
This would suggest that there is a relationship between melatonin and the sleep-wake cycle. However, it is not a casual relationship, other factors might be involved such as individual differences between the participants and the rest of the target population as well as it being down to simply a bad nights sleep previously.
Furthermore, the setting is high artificial meaning the study lacks real ecological validity. Nevertheless, the study suggests that it is mainly endogenous pacemakers that control the sleep-wake cycle more than the exogenous zeitgebers but are very much ignored in the studies. To combat this, Folkard (1985) investigated the importance of external cues by having 12 participants in isolation for three weeks with a strict schedule of sleep.
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Over time, the clock with the participants quickened its pace so that they had it was two hours fast. With only one participant conforming to the clock change, suggests that exogenous zeitgebers have a limited effect on the sleep-wake cycle but nonetheless, it still has a marginal effect. What the studies do not take into account is the effect of the light on the sleep-wake cycle, which has a more convincing effect on the sleep-wake cycle than the clock in Folkard’s study.
Another biological rhythm is infradian rhythms such as hibernation, seasonal affective disorder (SAD) and the menstrual cycle. Some infradian rhythms are greatly influenced by exogenous zeitgebers. For example, seasonal affective disorder is a cycle that happens once a year during the winter months that cause certain types of people to become clinically depressed.
The link between SAD and melatonin and serotonin is that as there is less light in the winter, more melatonin is produced making someone less active as well as the levels of serotonin then being reduced, thus making someone unhappy. However, this does not happen to everyone due to the individual differences with the amount of melatonin that is produced.
However, though some infradian rhythms suggest a strong link between exogenous zeitgebers and the cycle, others show a stronger link between endogenous pacemakers and the cycle. For example, mainly the hormones that are created in the hypothalamus, pituitary gland and ovaries – such as GNRH, FSH and LH, respectively, control the menstrual cycle.
It is the fluctuations in these hormones that affect the ovulation and even the sexual behaviour; therefore, this suggests that the endogenous pacemakers have a great deal of influence on the menstrual cycle and even the behaviour of a woman. However, the hormone pheromone, though an endogenous pacemaker, can prove to be an exogenous zeitgeber and have an effect of other women’s menstrual cycle. Though pheromones are biochemical substances, they can be released into the air and affect others if they are around them too often.
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Russell et al. (1980) found by having one woman having pads under her arms and having a group of sexually inactive women faced with her pheromones on their upper lip had an impact on the menstrual cycles. Using a control group, Russell found that after five months, four out of the five women in the experimental group had synchronised menstrual cycles to within a day of the odour donor. These results coincided with other researchers such as McClintock & Stern (1998) and can be explained by an evolutionary approach.
The pheromones that the women were exposed to caused the women to ovulate at the same time, this would suggest they would all have an equal chance of getting pregnant and if one did, there are more people to look after it, giving them a larger chance of survival. Although, there are questions that remain, such as the extent that a woman’s menstrual cycle can become synchronised is still not exactly clear. Neither is there is a clear reason why it must happen nor how it even happens.
With the evidence shown in both circadian and infradian rhythms, it can be invaluable in its application. For example, the research into the sleep-wake cycle can be applied to shift work and what is the best way to cope with this as well as being able to use this research in chronobiology. By learning more about the genes responsible for circadian rhythms will also improve our understanding of the human body, which would help search for more treatments into sleep disorders like insomnia.
In conclusion, the research seems to suggest that the infradian rhythms and circadian rhythms are greatly influenced by the endogenous pacemakers such as the hormones involved. However, the fact that exogenous zeitgebers such as light and pheromones can have an effect suggest that researchers have only made a window into biological rhythms as there is so much more to know and until that point, no undoubted conclusion can be drawn.
