The Effect of Endogenous Pacemakers & Exogenous Zeitgebers on the Sleep/Wake Cycle Flashcards
What is the SCN?
The supra-chiasmatic nucleus of the hypothalamus is the main endogenous pacemaker in mammals & is responsible for internally controlling the sleep/wake cycle. The SCN is located above the optic chiasm, which is the point where the optic nerve from each eye crosses over. The position of the SCN means it can receive neural signals about the amount of light.
Explain the role of melatonin on the sleep/wake cycle.
- Neural signals from the SCN are transmitted to the pineal gland, located just behind the hypothalamus.
- The pineal gland secretes melatonin, which is a hormone that inhibits neural mechanisms that promote wakefulness.
- When it’s dark, the pineal gland increases the production of melatonin & a person will start to sleep.
- In the morning, when it’s light, secretion of melatonin from the pineal gland decreases so a person feels more awake.
Evaluation of the Effect of Endogenous Pacemakers.
- Supporting evidence for the role of endogenous pacemakers in the sleep wake cycle: French cave explorer, Michael Siffre (1975) who spent 205 days underground in a cave & lacked any clues about when it was day/night.
- It is biologically deterministic to assume that our sleep/wake cycle is controlled by endogenous pacemakers: compared to other species, the pre-frontal cortex of the human brain offers the ability to suppress such biological urges. Ultimately, the theory may be considered to be an example of soft determinism.
- An advantage of research into endogenous pacemakers is the practical application to combat jet lag: this is especially useful for people travelling from west to east.
- Evidence supporting the role of the SCN as an endogenous pacemaker: Morgan (1995) who selectively bred a strain of hamsters w/ an abnormal circadian rhythm of 20 hours. SCN neurons were transplanted into normal hamsters (w/ a biological rhythm of 24hrs), which then displayed the same abnormal rhythm. This provides strong evidence for the importance of the SCN in maintaining biological rhythms.
What is the dominant exogenous zeitgeber in humans?
Light. Research has shown the exposure to bright light can slow down melatonin production. SCN receptors are sensitive to neural signals about changes in light levels & use this info to synchronise the internal activity of the body’s organs & glands w/ the external environment. Light therefore resets the biological clock each day, keeping it on a 24hr activation cycle.
What serves as additional exogenous zeitgebers?
Social cues such as set meal times. This is evident when travelling long distances across time zones; if an individual adopts the meal times of their destination in the days prior to departure, this can help entrain their biological clock & lessen the symptoms of jet lag upon arrival.
Evaluation of the Effect of Exogenous Zeitgebers.
- Supporting evidence for the role of exogenous zeitgebers in the sleep/wake cycle: Vetter et al. (2011). An experimental group of office workers were exposed to brighter white light (8000K-cool hue), whilst a control group were exposed to regular white lighting (4000K-warm hue).
- It is environmental reductionist to focus on light as an exogenous zeitgeber in the sleep/wake cycle: the majority of research suggests that light is the dominant exogenous zeitgeber in humans. However, due to the complex contribution of many exogenous zeitgebers on the sleep/wake cycle, it’s problematic to investigate singular mechanisms involved in isolation. E.g. Campbell & Murphy’s study.
- Supporting evidence for the role of exogenous zeitgebers in the sleep/wake cycle: Campbell & Muprhy’s study (1998). They measured the effect of light shone behind the knee on body temp & melatonin concentrations.
- An advantage of research into exogenous zeitgebers is the practical application to mobile phone use: the bright white light (cool hue) used by LED mobile phone screens can reset SCN activity which reduces melatonin production & prevents a person from becoming sleepy.
