10. Effect of endogenous and exogenous factors on the sleep/wake cycle

what are endogenous pacemakers?

Internal body clock that sets many of our bodily rhythms, including sleep

what is the main endogenous pacemaker which effects when we sleep and when we awake?

Superchiasmatic nucleus (SCN)

What is the superchiasmatic nucleus?

A bundle of nerves located in the hypothalamus in the brain

What are exogenous zeitgebers?

External cues that have an influence on when we sleep or awake, e.g. light

how do endogenous pacemakers and exogenous zeitgebers influence our circadian rhythm?

• the main endogenous pacemaker is the superchiasmatic nucleus.

• The SCN is located above the optic area. This means that it can receive information about light directly. The SCN passes the information about day length/light to the pineal gland.

• Based on this information, the pineal gland will release melatonin

• During the night, the pineal gland increases melatonin production. With more daylight, less melatonin. Therefore, the SCN is regulated by light from our outside world.

• However, even in the absence of light the SCN generates a rhythm related to its production of protein. When it reaches a certain level of protein it passes a message to the pineal gland and melatonin will be released or inhibited.

• So although daylight influences the SCN it’s not absolutely essential

Research supporting the SCN- Morgan

• hamsters were bred to have a circadian rhythm of 20hrs

• their SCN was removed and transplanted into a hamster with a normal SCN

• the normal hamsters who had the SCN transplanted adopted this same irregular 20hr circadian rhythm

• when hamsters with nocturnal patterns of activity (normal) had their SCNs replaced with SCNs from mutated hamsters who slept through the night and were active during the day, the hamsters followed the new daytime activities of the donor hamsters

• This shows how essential the SCN is for circadian rhythm and how endogenous pacemakers are important for biological rhythms

Evidence against SCN- exogenous zeitgebers

Light is the most influential exogenous zeitgeber, it is an important factor in our environment that resets our biological clock, called entrainment

Light enters the eyes through the retina and this information is passed onto the SCN

although most of the processes are internally driven, it can be governed by light

how do low levels of light impact circadian rhythm

• low levels of light (retina)

• Via the optic area to the SCN

• SCN sends signals to pineal gland

• pineal gland releases melatonin

• induces sleep

research supporting exogenous zeitgebers- Campbell and Murphy

• light can be detected by skin receptors on the body, even when the same information is not received by the eyes

• 15 participants were woken at various intervals and light pads were shone on the back of their knees

• found they could shift the sleep-wake cycle, a change in sleep wake cycle of up to 3hrs in some cases

• This shows that light can influence the brain without reflecting on the eyes- light can entrain through eyes and skin. Light is a powerful exogenous zeitgeber that doesn’t need to rely on eyes to exert influence on the brain

AO3 to consider- animal studies

Generalising findings from animal studies to humans is questionable

AO3 to consider- methodological issues in research

The findings from Campbell and Murphy study have yet to be replicated

Critics have suggested that participants may have been exposed to a limited amount of light to their eyes which would be a major confounding variable and affect the validity of the results.