Endogenous Pacemakers & Exogenous Zeitgebers

Question 1

Endogenous Pacemakers:

Answer 1

• EP - pacemakers within our body
• Most important EP is the suprachiamsatic nucleus (SCN) in the hypothalamus
  SCN = main endogenous pacemaker (master clock) + is responsible for the control of the sleep/wake cycle + other circadian rhythms including body temperature and heart rate
• SCN has built in circadian rhythm - resets when external light levels change
• SCN receives information from optic nerve + regulates sleep by influencing activity of pineal gland

Question 2

SCN

Answer 2

suprachiamsatic nucleus (SCN) in the hypothalamus

Question 3

Exogenous zeitgebers

Answer 3

• EZ - influences outside of the body. Most dominant in terms of sleep/wake cycle is light
• Light resets the internal biological clock (SCN) every 24 hours
• By activating light detecting cells in the retina (cells which contain melanopsin)
• The optic nerve carries a signal to SCN and influences the pineal gland to reduce melatonin production and makes us feel awake and resetting the sleep/wake cycle.
• Leads to entrainmenet of human body alongside environment - helps us better adapt to the environment by influencing endogenous cycles

Question 4

Social cues:

Answer 4

• There are other EZ which control our biological rhythms, e.g. mealtime, clocks, noise
• While light is dominant EZ, individuals are able to compensate for absence of light by responding to social zeitbeigers

Question 5

Pineal gland/ (disruption of sleep/wake cycle):

Answer 5

• SCN regulates sleep cycle by communicating with pineal gland
• SCN sends signals to pineal gland = instructing it to increase secretion of melatonin at night (inducing sleep) aabd reduce it in the morning as light increases.
• The pineal gland and SCN collectively control the sleep/wake cycle but their activity must be synchronised with the light/dark cycle of outside world
• if SCN not in sync w/ outside world - disruption of sleep/wake cycle

Question 6

Evals for endogenous pacemakers:

Answer 6

• A strength for the role of EP: support from animal studies for the role of SCN as the main EP.
• A strength for the role of EP: case study support.

Question 7

A strength for the role of EP: support from animal studies for the role of SCN as the main EP.

Answer 7

Morgan: removed SCN from hamsters and found that their circadian rhythm disappeared. In addition to this: when hamsters are bred (with a circadian rhythm of 20 hrs rather than 24 hrs) and their SCN’s are then transplanted into normal hamsters, the normal hamster will display the mutant rhythms.
Since changes in the SCN led to changes in circadian rhythm, this provides strong evidence for the importance of the SCN controlling and maintaining circadian rhythm.
This adds validity to research into EP

Question 8

A strength for the role of EP: case study support.

Answer 8

The French cave explorer, MIchael Siffre: spent 7 months underground in a cave.
He was adequately fed, had opportunities for exercise and was able to make contact at all times by telephone, but he totally lacked any cues about when it was day or night. His sleep pattern settled down to just over 24 hours.
Therefore Siffre concluded that the 24 hour sleep cycle is internally caused by EP as his sleep/wake cycle persisted in the absence of external cues.
This shows that internal pacemakers are responsible for maintaining the 24hr sleep/wake cycle

Question 9

Evals for exogenous zeitgebers:

Answer 9

• A strength of role of EZ: research support
• A strength of role of EZ: research support for the effects of diff types of light.

Question 10

A strength of role of EZ: research support

Answer 10

Burgess: found that when ppts exposed to continuous bright light right after they woke up- their circadian rhythms shifted by 2.1 hours over the course of their study. As a result, ppts felt sleepier 2 hours earlier in the evening and were better adjusted to the local time at their destination after a long flight.
This demonstrates the role of light in circadian rhythms as it shows changes of external light can lead to changes in the sleep/wake cycle. This happens despite no direct changes being made to the endogenous pacemaker- shows that EZ are a key influence on the sleep/wake cycle.

Question 11

A strength of role of EZ: research support for the effects of diff types of light.

Answer 11

Vetter et al: compared the influence of light change on sleep timing by exposing one group to working under warmer light (4000 K) and another group to working under blue-enriched light (8000 K) in the office for 5 weeks. The warmer light group sleep patterns remained entrained to natural sunrise and dawn, but the blue-enriched light sleep patterns synchronised with office hours instead.
Shows that light is a dominant zeitgeber as changes in light leads to changes in sleep pattern and entrainment.
This was seen as the more intensive the light is, the more likely we are to entrain to it.
Validates our understanding of light as an EZ

Question 12

Disruption in sleep rhythms can be due to endogenous pacemakers and exogenous zeitgbeirs being out sync. examples of how they could become out of sync:

Answer 12

• shift work: when someone works day shifts for a fixed duration and then has to shift to night shifts due to a shift rota
• jet lag: when someone travels to another time zone and this may disrupt their natural sleeping time as they have to adapt to the sleeping times in that country

Question 13

Disruption in sleep rhythms can have the following BEHAVIOURAL effects:

Answer 13

• lowered productivity
• tiredness
• unable to focus

Question 14

Disruption in sleep rhythms can have the following PSYCHOLOGICAL effects:

Answer 14

• easily irritable
• depression

Question 15

Disruption in sleep rhythms can have the following PHYSIOLOGICAL effects:

Answer 15

• high blood pressure
• heart disease vulnerability