Control of the sleep/wake cycle Flashcards
Endogenous pacemakers
Internal body clocks that regulate many of our biological rhythms such as the influence of the SCN on the sleep/wake cycle
Exogenous zeitgebers
External factors that affect or etrain our biological rhythms such as the influence of light on the sleep/wake cycle
Sleep/wake cycle
A daily cycle of biological activity based on a 24 hr period (circadian rhythm) that is influenced by regular variations in the environment such as the alternation of night and day
What is the suprachiasmatic nucleus (SCN)?
A cluster of cells that control the sleep/wake cycle
Nerve fibres from the eye cross at the optic chiasm on the way to the right and left visual areas.
SCN lies just above the optic chiasm and receives info about light from this structure
SCN monitors light levels via optic chiasm and alters sleep/wake cycle to synchronise to the world
Case of DeCoursey
Case of Ralph et al
Destroyed SCN connections in the brains of 30 chipmunks which were returned to their natural habitat and observed for 80 days. Their sleep/wake cycle disappeared and many were killed by predators
Ralph et al bred ‘mutant’ hamsters with a 20 hour sleep/wake cycle. SCN cells were transplanted from the foetal tissue of these hamsters into the brains of normal hamsters, which then developed cycles of 20 hours
Pineal gland and melatonin are endogenous mechanisms
- explain the process
SCN passes info on day length to the pineal which increase production of melatonin during the night
Melatonin - hormone that induces sleep and is inhibited during periods of wakefulness. It has also been suggested as a casual factor in S.A.D
What is an optic chiasm
SCN monitors light levels via optic chiasm. When the light level drops the SCN changes the level of melatonin the the body via pineal gland
More melatonin = more sleepy
Less melatonin = less sleepy
Evidence suggesting the importance of light (exogenous zeitgeber)
Light can reset the body’s main endogenous pacemaker and also has a direct influence on key processes in the body controlling hormone secretion, blood circulation e.t.c
Campbell and Murphy - Woke 15 participants at various time and shone a light on the backs of their knees - producing a deviation in the sleep/wake cycle of up to 3 hrs. Light does not necessarily rely on the eyes to influence SCN
Social cues - some people have argued that social factors like alarm clocks and parenting habits are an important additional zeitgeber
EVALUATION: Flawed research
The use of hamsters in DeCoursey’s research is problematic. Because of our biological distinctions and the wide range of contextual situations in which we exist, humans would react quite differently to changes of their biological cycles.
As a result of this, studies conducted on other species are unable to clarify how endogenous pacemakers function in human biological processes.
Conclusion (overall argument)
The role of exogenous zeitgebers and endogenous pacemakers has been shown by numerous studies; yet, the argument may still be viewed as biologically reductionist. The behaviourist perspective, for instance, would argue that social norms and other people have an impact on body rhythms.
For instance, sleep happens during the dark since it is the social norm and it is not socially acceptable for someone to go about their everyday business at night. Since the research presented here only takes into account one biological mechanism and ignores the other, very different perspectives, it could be accused for being reductionist.
EVALUATION: Ethics
Since many of the mechanisms underlying the sleep-wake cycle are similar in many species, research on animals is required in this area. Since the brains of mammals share similarities, it is possible to draw generalisations about the human brain based on the presence of a SCN and pineal gland, for example, in the brains of chipmunks and hamsters.
The ethical issues around this kind of research, however, are more serious. When placed back in their normal environment, the animals in the DeCoursey et al. study were put at risk, and the majority of them perished as a result.
EVALUATION: Age related insomnia
Research indicates that as people age, their sleep quality declines. This might result from aging-related natural shifts in the circadian rhythm, which cause earlier bedtimes and interrupted sleep at night (Duffy et al, 2015).
Nonetheless, research indicates that external variables could be accountable for the changes in sleep habits observed in the elderly population.
According to research by Bernadette Hood et al. (2004), elderly adults who were typically more active and who received more natural light had better control over their sleeplessness.
EVALUATION: Environmental observations
A limitation is that exogenous zeitgebers don’t always work the same way in different settings. The experiences of those who live in areas with minimal summer darkness and minimal winter light provide a different perspective from the mainstream narrative.
For example, it’s reported that the Inuit people living around the Arctic Circle have consistent sleep habits throughout the year, even though they spend around six months in nearly complete darkness.
This implies that endogenous pacemakers, which are capable of overcoming external variations in light, are the primary regulators of the sleep-wake cycle.
