A2 Only: Biological rhythms: circadian, infradian and ultradian and the difference between these rhythms. The effect of endogenous pacemakers and exogenous zeitgebers on the sleep/wake cycle.
Biological rhythms are classified as circadian, infradian and ultradian. A circadian rhythms is one lasting ‘about one day’, such as the sleep-wake cycle, body temperature or urine production. These rhythms allow animals to prepare for predictable daily environmental changes, such as night and day. Infradian rhythms are those which take place over longer than a day (i.e. monthly or seasonally) such as the menstrual cycle, hibernation and seasonal affective disorder. Ultradian rhythms are those shorter than a day, such as REM/nREM cycles in sleep and a proposed ‘Basic-Rest Activity Cycle’, of which research remains inconclusive.
Research is interested in whether these cycles are controlled by internal mechanisms (endogenous pacemakers) or external cues, such as light and weather (exogenous zeitgebers). A whole host of interesting studies have been conducted, which try to isolate endogenous pacemakers, by controlling and manipulating the external environment. Research involving human participants has focused on depriving them of possible zeitgebers (‘time-givers’) like sunrise and sunset, temperature changes during a 24 hour period and wristwatches! Participants tend to maintain a cyclical rhythm but it extends to about 25 hours (Siffre, 1975). So, endogenous pacemakers can keep a rhythm but exogenous zeitgebers are needed to stick to a 24 hour rhythm.
Endogenous Pacemakers & Exogenous Zeitgebers
The main endogenous pacemaker, which has been identified as controlling circadian rhythms, is the suprachiasmatic nucleus (SCN), located in the hypothalamus. It is a bundle of nerves with an inbuilt circadian rhythm. Scientists who isolated cells from the SCN discovered that they beat to a roughly 24 hour rhythm, even outside of the human body. The SCN is connected to the optic nerve and is therefore directly affected by light levels (an exogenous zeitgeber) When light hits the retina of the eye, an action potential sends the message along the optic nerve, which subsequently regulates melatonin secretion form the pineal gland. The pineal gland in the brain converts the neurotransmitter serotonin into the hormone melatonin. Light therefore inhibits the release of melatonin; a hormone which makes you feel sleepy.
Interesting, human case studies into circadian rhythms in the absence of exogenous zeitgebers; particularly light (and therefore a free-running clock) indicate that the brain’s day would be more like 25 hours long. Light is a very important zeitgeber – flashes of light are enough to ‘reset’ the internal clocks of animals living in the dark (Aschoff, 1979). One blind man needed to take stimulants and tranquilizing drugs to maintain a 24 hour cycle!
Evaluating Biological Rhythms
Evolutionary Approach: There is a clear evolutionary advantage to the entrainment of the sleep-wake cycle through exogenous zeitgebers. This adaption allows us, and other mammals, to adapt to our changing environment through the seasons, as well as across time zones. This ability increases our survival chances, as seen by Decoursey et al.‘s (2000) study into the link between a damaged SCN and survival in the wild.
Animal Research: There are a number of problems with using animal studies in this area. The first problem is the harm to the animals concerned. However, there is a costs-benefits consideration to research with non-human animals. The costs to the animals (e.g. Decoursey’s chipmunks were made more vulnerable to predators as a result of being used in the study) can be balanced out by the possible benefits of understanding the important role played by the SCN. There is also the issue of generalisation to humans. It is problematic to generalise animal findings direct to human beings because of differences in the biological systems of different species. For example, in reptiles and birds, light acts directly on the pineal gland through the skull, whereas in humans, this process is mediated by the SCN. This questions the value of findings from animal studies. For ethical reasons it is difficult to carry out the same experiments on human beings (i.e. removing the SCN in order to study the effect on circadian rhythms) as this would cause physical harm to human participants.
Real life: Today, sleeping habits in technologically advanced societies are not determined by darkness. We do not normally go to bed at dusk (especially in winter when, in the UK, dusk can be mid-afternoon). Before the invention of the electric light in the late 19th century, sleep/waking for the majority of people would have been determined by light and darkness. Changes in the range of zeitgebers have been relatively recent in human social evolution.
Cultural differences: In some societies it is not possible to use light and dark as zeitgebers. The Inuit in Greenland have periods in the year of 24 hours of light or 24 hours of darkness. Yet they maintain normal 24-hour sleep/waking cycles. For them, social and work habits, rather than light, are used to synchronise the sleep/waking endogenous pacemaker with the outside world.
Nature/Nurture debate: Biological clocks are innate mechanisms with a clear genetic component, which emphasises nature over nurture; it is difficult for us to change our sleep/waking pattern.
Reductionism: Explanations of biological rhythms are also reductionist as they explain them at the lowest level of physiological mechanisms, sometimes underestimating the contribution of higher-level factors such as social zeitgebers.