Lesson 14 - Endogenous pacemakers and exogenous zeitgebers in the sleep wake cycle Flashcards
Endogenous pacemakers
The body’s internal biological clock
Exogenous Zeitgebers
External cues for regulation
SCN
The Suprachiasmatic Nuclei is a cluster of tiny nerve cells in the hypothalamus. It acts as the master clock and coordinates other regions involved in the sleep-wake cycle.
The SCN receives information about light from the optic nerve
It also regulates the production of melatonin in the pineal gland
Strengths of endogenous pacemakers
- Morgan (1995) studied a breed of hamster who had an unusual body clock of 20 hours. The SCN neurons from these peculiar hamsters were transferred into the brains of normal hamsters. They also developed a sleep cycle of 20 hours. The SCN neurons of the normal hamsters was transferred into the abnormal breed and they showed a normal circadian cycle of 24 hours. Therefore endogenous pacemakers like the SCN are vitally important
- There are animal studies to support the SCN. DeCoursey et al (2000) destroyed the SCN connections in 30 chipmunks and returned them to the wild where they were observed for 80 days. Their sleep cycle was gone and most of them were killed by predators, possibly by being awake and vulnerable to attack.
- Folkard (1996) studied a student, Kate Aldcroft, who spent 25 days in a lab with no exogenous zeitgebers, but her core temp rhythm was still at 24 hours, showing the importance of internal clocks.
Weaknesses of the SCN
- DeCoursey’s study has very obvious ethical issues, so this limits the repeatability of the study and it also limits the ecological validity of the study, as animals do not have their brains tampered with
- Folkard also found that Aldcroft’s sleep-wake cycle extended to 30 hours, and she slept for as long as 16 hours, which shows that exogenous zeitgebers may be more important.
Light
- Receptors in the SCN are sensitive to light changes. Melanopsin, a protein in the retina which is sensitive to light is important in maintaining the 24 hour cycle
- Jet lag can occur when your inbuilt clock tries to impose itself, but out of synchrony with your new environment, like when you are travelling
Social cues
- Another example of exogenous zeitgebers
- In the absence of light, research has found that individuals can compensate for this with social cues, like mealtimes
- People also thought that social cues were the primary zeitgeber for blind people, but this is still better explained with light, as you do not necessarily need to be sighted to interact with light
- Imposing mealtimes also has a very important role in shaping the cycles of infants. Their cycle begins at around 6 weeks, and by 16 weeks, they have some sort of routine.
- Adapting to local times of mealtimes and bedtimes is also a good way to combat jet lag.
Strengths of exogenous zeitgebers
- The role of melanopsin comes from studies of blind people. Skene and Arendt (2007) found that for the majority of blind subjects who have some form of light perception still have normal circadian rhythms. Therefore the pathway from the retina to melanopsin is still intact.
- Burgess et al (2003) also found that exposure to bright light before an east-west flight decreased the time needed to readjust to the local time. Therefore light is extremely influential on the internal body clock
Weaknesses of exogenous zeitgebers
Not all lights have the same influence on the sleep wake cycle and it must be taken with caution when generalising all studies about light. Vetter et al (2011) kept one group of participants in a room with warm artificial light and another in a room with artificial blue light for a five week period. The participants in warm light synchronised their rhythms to the natural dawn, but the group in blue light synchronised their rhythms to office hours. Therefore natural light is more influential on the sleep cycle
