Endogenous pacemakers and exogenous zeitgebers

What are endogenous pacemakers?

Internal body clocks that regulate biological rhythms from within the body.

What is the SCN, explain its relation with light and how it regulates the sleep wake cycle.

The SCN is a bundle of nerve cells found in the hypothalamus, its the body’s main endogenous pacemaker. It receives light ( exogenous zeitgeber) signals from the retina via the optic nerve. Once it gets dark the SCN will signal the pineal gland to release melatonin which will release melatonin, inducing sleep. As it gets light again, melatonin is inhibited to promote alertness. The exogenous zeitgeber light aids the SCN in keeping aligned with the 24 hour day night cycle.

Who found evidence of the SCN ?

Morgan 1955 Bred mutant hamsters with a 20 hr circadian rhythm and transplanted their SCN into normal hamsters, these hamsters adopted the 20 hr rhythms

Who found evidence of the role of light as an exogenous zeitgeber?

Touitou et al 2017

Found that exposure to blue light from screens suppresses melatonin production in adolescents leading to delayed sleep and disrupted circadian rhythms. Shows that artificial light is an exogenous zeitgeber.

What is an exogenous zeitgeber?

External cues that help regulate the body’s internal biological clock

How are social cues an exogenous zeitgeber? Research

Wegmann and Klein 1981, studied jet lag and shift work, found that circadian rhythms can adjust to new time zones, but adjustment is slow. The presence of social cues help but the most effective zeitgeber for entrainment is light. Social cues have some influence but are not strong enough on their own.

Evaluate endogenous pacemakers

Strengths

Role of SCN

Point; Research supports the role of the SCN as a crucial endogenous pacemaker.

Evidence; Morgan 1995 removed the SCN of hamsters and found that their circadian rhythms disappeared. When SCN's were transplanted back into the hamsters, their circadian rhythms returned.

Explain; Provides strong experimental evidence that the SCN controls the body's circadian rhythm.

Link; Therefore, supporting the idea that the SCN plays a central role in regulating internal biological clocks.

A; Animal study - doesn't account for the complexities of human behaviour. - scientifically questionable. - limits external validity.

Evaluate endogenous pacemakers

Strengths

Dangers of disrupted rhythms

Point; Disruption to endogenous pacemakers, e.g. SCN, can have negative effects.

Evidence; Touitou et al 2017 found that the blue light from screens, delays melatonin production in adolescents, disrupting their circadian rhythms and causing sleep deprivation.

Explain; Shows how sensitive the SCN is, and how disruption can impact mood, cognition, and overall health.

Link;Proper functioning of endogenous pacemakers is vital for physical and psychological health.

Evaluate exogenous zeitgebers

Strengths

Using light to avoid jet lag

Point: Research supports the role of light as a powerful exogenous zeitgeber regulating the sleep wake cycle.

Evidence: Burgess et all 2003, found that exposure to bright light in the morning (light therapy), helps travellers adjust to new time zones - reducing the effects of jet lag.

Explain; Participants exposed to strong light in the morning shifted their circadian rhythms nearly 2 hours early, allowing better adaptation to an eastward flight.

Link; Supports the practical application of light exposure as a tool for entraining biological rhythms and reducing jet lag.

Evaluate exogenous zeitgebers

Limitation

Light is important but not necessary

Point; Research shows that exogenous zeitgebers are important but not essential.

Evidence: Aschoff et al 1971, placed participants in a bunker with no natural light or time cues, despite this participants maintained a sleep wake cycle close to 24 - 25 hours.

Explain; Demonstrates that while circadian rhythms can persist without light, they are not perfectly accurate and need exogenous zeitgebers like sunlight to stay aligned with the 24 hour day.

Link; Therefore the study supports the idea that light plays a key role in entraining circadian rhythms though endogenous pacemakers can still operate in its absence.