endogenous and exogenous Flashcards
sleep/wake cycle defintion
A daily cycle of biological activity based on a 24-hour period (circadian rhythm) that is influenced by regular variations in the environment, such as the alternation of night and day.
Endogenous pacemaker definition
Mechanisms within the body that govern the internal, biological bodily rhythms.
What is the primary pacemaker in humans
The SCN
Where is the SCN located
the hypothalamus
What does the SCN do
The SCN is influential in maintaining circadian rhythms such as the sleep/wake cycle.
Nerve fibres connected to the eye cross in an area called the optic chiasm on their way to the left and right visual area of the cerebral cortex. The SCN lies just above the optic chiasm.
It receives information about light directly from this structure. This continues even when our eyes are closed, enabling the biological clock to adjust to changing patterns of daylight whilst we are asleep.
Passes info to pineal gland
The pineal gland
The SCN passes the information on day length and light that it receives to the pineal gland.
This is another endogenous mechanism guiding the sleep/wake cycle.
During the night, the pineal gland increases production of melatonin - a chemical that induces sleep and is inhibited during periods of wakefulness.
Melatonin has also been suggested as a causal factor in SAD.
Research into endogenous pacemakers
Morgan (1995):
The researcher bred hamsters so that their circadian rhythms were 20 hours, rather than the usual 24-hour cycle.
SCN neurons from the 20-hour hamsters were transplanted into the brains of normal hamsters.
The hamsters with transplanted SCN neurons went on to follow the same irregular 20-hour circadian rhythm as the original 20-hour cycle hamsters
thus, the transplanted SCN had imposed its irregular pattern onto the hamsters
Researcher concluded that the SCN is a significant endogenous pacemaker which is crucial to the proper functioning of circadian rhythms
these findings also demonstrate the importance of exogenous zeitgebers to the sleep-wake cycle (endogenous pacemakers and exogenous zeitgebers work hand-in-hand)
Eval endogenous
str: internal validity
str: more research support
lim: animal study
lim: biologically reductionist
Endo str: int validity
The use of carefully controlled conditions and removal of the SCN means that this study has good internal validity
The researcher was able to directly observe the effect of circadian rhythm disruption in a way which would have been impossible with human research
Endo str: research support
Further support for the influence of endogenous pacemakers on circadian rhythms was provided by Skene & Arendt (2007)
They pointed out that most blind people who still have some light perception have normal circadian rhythms of 24 hours, whereas blind people with no light perception display irregular circadian rhythms
This demonstrates the importance of light in terms of the internal sleep-wake cycle
endo lim: animal study
Morgan’s research is flawed due to its use of hamsters as subjects
Humans are very different biologically, cognitively and socially from hamsters
This means that research using hamsters is not generalisable to humans
The role of endogenous pacemakers in humans therefore cannot be explained using animal research
endo lim: biologically reductionist
It is biologically reductionist to focus on a narrow range of explanations when considering something as complex and prone to variation as the patterns and habits involved in the sleep-wake cycle
There are likely to be an array of other factors which also contribute to circadian rhythms which may supersede the endogenous pacemaker explanation e.g. high-stress levels may interfere with regular sleep
some people can (and may frequently) fall asleep in bright light, or during daylight hours which means that light may not be as important to the sleep-wake cycle as some research suggests
Exogenous zeitgeber definition
An environmental cue, such as light, that helps to regulate the biological clock in an organism.
-any factors in the sleep-wake cycle which act as external cues to either wakefulness or sleepiness
What is the most important zeitgeber for animals
Light
How does light reset the internal clock
Specialized retinal cells with melanopsin send signals to the SCN based on light levels.
What does light do
resets the internal clock daily, ensuring it stays on a 24-hour cycle.
Social cues
Social cues like mealtimes or social activities also affect the circadian rhythm. Studies have shown that even without natural light, people can adjust to time zone changes by responding to social cues, as seen with jet lag or blind individuals, whose sleep-wake cycles are still influenced by light
Exogenous zeitgeber research
Campbell & Murphy (1998):
15 sleeping participants were woken at various intervals throughout the night
A light pad was shone onto the back of their knees
This resulted in irregular sleep-wake cycles for the participants (as high as three hours deviation for some of them)
The researchers concluded that light is a key exogenous zeitgeber - even when it shines on the back of the knee rather than being detected by the eye
eval exogenous:
lim: sample size
lim: not replicated
lim:case study into social cues
exogenous lim: sample size
A sample size of 15 is very small
This means that the researchers are limited as to how generalisable and robust their findings are
Generalisability is limited due to the unrepresentative nature of a small sample
The robustness of data is limited as small numbers lack statistical power
exogenous lim: not replicated
Campbell & Murphy’s findings have not been replicated in any subsequent studies
If a study cannot be replicated according to a standardised procedure and conditions then it cannot be checked for consistency
Not being able to check for consistency of findings means a lack of reliability
exogenous lim: social cues case study
Evidence that challenges the role of exogenous zeitgebers.
Laughton Miles et al, 1977, recount the study of a young man, blind from birth, who had an abnormal circadian rhythm of 24.9 hours. Despite exposure to social cues, such as regular mealtimes, his sleep/wake cycle could not be adjusted. This suggests that social cues alone are not effective in resetting the biological rhythm.
