Circadian Rhythms: the sleep/wake cycle Flashcards
Discuss research studies into circadian rhythms: the case study of siffre
Spent 6 months inside a cave in Texas. There were no zeitgebers such as natural light or sounds. he had no contact with the outside world via telephone, but had no idea what time it was. His behaviour e.g. when he slept/woke and when he ate his meals was monitored. when he was awake he put the lights on and when he went to bed, he turned the lights off.
His sleep/waking cycle was erratic at first but then settled into a fairly regular cycle of about 25 hours i.e. slightly longer than the 24 hour day. when he emerged the 179th day it was only his 151st day!
Evaluation:
- the data collected is reasonably objective (records of timing of sleep/waking etc) so easy to check
- It is a case study, so generalisation to other people is unsafe; however other research (e.g. wever whose participants spent several months in a bunker) show similar findings (maintenance of a circadian rhythm which had extended to between 25 and 27 hours)
The fact that a sleep/waking rhythm developed in the absence of exogenous zeitgebers suggests that there are internal processes governing the sleep/wake cycle. The fact that the rhythm was not 24 hours suggests that under normal conditions the endogenours pacemaker is re-set by external cues. It seems that our internal clock must have a 25 hours cycle and that zeitgebers must reset the clock to the usual 24-hour cycle. This explains why people find it easier to stay up an hour later, than go to sleep an hour earlier.
The most likely exogenous zeitgeber for the sleep/waking cycle is light.
Discuss Research studies into circadian rhythms: Morgan
Evidence for the role of SCN as an endogenous pacemaker for circadian rhythms comes from studies of hamsters (e.g. Silver 1966, Morgan et al. 1995). They firstly established the presence of circadian rhythms in a group of hamsters by recording their sleep/wake cycle over a period of time. They then removed their SCN. the hamsters still ate, slept etc, but not at regular times - their circadian rhythms disappeared. Later they transplanted SCN tissue from hamster foetuses, and found tht after a few days the circadian rhythms re-established. Morgan also transplanted SCN cells from hamsters which had been bred to have longer or shorted circadian rhythms. they found that after transplant, the hamsters took on the rhythms of the donor hamsters.
Evaluation:
These studies have provided extremely valuable evidence to suggest that the SCN is the biological clock which controls the sleep/waking cycle. Carefully controlled scientific procedures were used which have been replicated by other researchers, meaning we can have much confidence in these findings. They present a good example of the way psychology can use highly scientific methods. However, many critics of animal studies argue that it is not possible to generalise from animals to humans. This represents the behavioural discontinuity view i.e. that we are qualitatively different to animals, and the same processes may not apply to humans. However, this research focuses on the most primitive of brain structures which are common to all mammals and although our cortex can modify its effects, much human behaviour is probably still largely controlled by the hypothalamus.
Some argue that there is the possibility that the observed effects of removing the SCN could partly be due to the trauma of surgery or damage to other areas of the brain. However, the fact that a normal circadian rhythm can be re-established with transplanted foetal tissue tends to discredit this view.
There are ethical concerns in using animals for this type of research. Although there are possible benefits for humans (e.g. treatments for SAD, jet lag and sleep disorder) it is ethically questionable subject one species to painful procedures for the benefit of humans. Some would argue that if they may lead to the relief of human suffering then it would be unethical not to carry out such studies.
This research suggests the SCN is a vital endogenous pacemaker, responsible for controlling circadian rhythms.
Explain the role of endogenous pacemakers and of exogenous zeitgebers
Support for the role of endogenous pacemakers has shown two important endogenous pacemakers: The SUPRACHIASMATIC NUCLEUS and the PINEAL GLAND. These function together as biological clocks in the brain, but it is likely that other mechanisms are involved too.
Explain the Suprachiasmatic nucleus
In mammals, the main endogenous pacemaker is in the SCN, a pair of tiny clusters of nerve cells in the hypothalamus which have an inbuilt circadian rhythmic firing pattern. This rhythm works by cells producing a protein for a number of hours until the protein level is high enough that it inhibits further production. For a number of hours the protein level drops until it crosses a threshold where the SCN starts producing it again.
The SCN sit just above where the optic nerves from each eye cross over (the optic chiasm). The SCN gets information on light from the optic nerve, even when our eyes are shut, as special photoreceptors in the eyes pick up light signals and carry them to the SCN. if our endogenous clock is running slow (such as an earlier sun rise) the morning light automatically shifts the clock ahead putting the rhythm in step with the world outside. This is the mechanism by which light re-sets the circadian clock in the SCN. Research by Morgan supports the role of the SCN as an endogenous pacemaker.
Explain the pineal gland and melatonin
The pineal gland is another endogenous pacemaker in the brain. IT converts the neurotransmitter serotonin into the hormone melatonin. It contains light sensitive cells. When the level of light falls, melatonin is produced by the pineal gland. Melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness. When light levels are high, the production of melatonin in the pineal gland is inhibited and we wake up.
Explain the role of light
even though endogenous mechanisms can control sleep/waking cycles in the absence of light (Siffre/Wever), light as an exogenous zeitgeber is necessary to reset the clock every day so that the biological rhythm is coordinated with the external world. A powerful example of the effects of lack of light was reported by Miles et al. A man blind from birth (therefore lacking the zeitgeber of light) had a circadian rhythm of 24.9 hours which he found difficult to modify despite exposure to clocks and social cues. he had to take stimulants and sedatives in order to get his biological rhythms in time with a 24 hour day.
AID - real life application of research into circadian rhythms
Knowing about how the circadian rhythm affects digestion, cortisol levels, heart rate etc is important when timing blood and urine tests (e.g. cortisol is much higher in the morning) and should be taken into account when taking many types of drugs. Chronotherapeutics is the study of how timing affects drug treatments. It is known that taking asprin to avoid heart attacks (which normally occur in the morning) is most effective at around 11pm. This is clearly an extremely useful (potentially life saving) application of such research. Alertness peaks in the morning and early evening, so this is the best time to study!
Understanding the way in which melatonin is produced in response to falling light levels also has important real life application to situations when lack of sleep may be an issue. For example, those who are suffering from jet lg are advised to spend time in sunlight to re-set their SCN to a new time zone, and babies who spend time in natural daylight may establish better night time sleeping patterns. This is important as lack of sleep can have serious consequences for new parents and for those who travel across time zones regularly. It is also possible to take melatonin in tablet form to help in these situations.
