Circadian rhythms

What is a circadian rhythm?

A biological rhythm that lasts about 24 hrs. Influenced by endogenous pacemakers and exogenous zeitgebers.

The sleep wake cycle

What are the exogenous zeitgebers and endogenous pacemakers involved?

Endogenous pacemakers;

• The SCN, a bundle of nerves in the hypothalamus, master clock. Receives signals from the retina and sends signals to the pineal gland, which releases the hormone melatonin which induces sleep.

Exogenous zeitgebers;

• Light, helps reset the SCN each day. Photoreceptors in the retina detect light and send signals to the SCN.

• Social cues, temperature.

Evidence for 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 rhythm.

What times do we experience dips in sleep drives?

2 - 4 am - Lowest alertness, high sleep pressure

1 - 3 pm - Post lunch dip, experience drowsiness.

9 - 11 am & 4 - 6 pm - peaks in alertness and cognitive performance.

What is homeostatic sleep control and what does it do?

It’s a process that balances sleep and wakefulness.

If you’re awake for longer - greater homeostatic sleep drive (sleep pressure), it lowers when your asleep and you’re likely to wake up when its low enough.

If you stay awake all night your homeostatic pressure increases, but if its day time the circadian altering signal may temporarily keep you awake, until sleep pressure overrides it.

Core body temperature - circadian rhythm

At what times is it highest/lowest

Lowest - 4:30 am - 36C - low alertness

Highest - 6pm - 38C - high alertness

A small drip in body temperature at 2pm to 4pm can help explain why we feel sleepy.

How is the production of melatonin a circadian rhythm?

Melatonin produced by the pineal gland is regulated by the SCN, starts to increase at around 9pm and falls towards the morning, it makes us feel sleep, playing a crucial role in regulating the sleep wake cycle.

Who spent time in a cave and what’d they find?

Michel Siffre

Spent 2 months in a cave without natural light, high sleep wake cycle extended to 24 hrs and 30 mins. He sleep, ate a woke according to his own internal clock. He concluded that there is a free running circadian rhythm showing the existence of endogenous pacemakers that operate without exogenous zeitgebers.

Who challenged Siffre’s findings?

Duffy et al 2001

Suggested that people vary between either morning or evening people, and therefore circadian rhythms aren’t universal, individual differences must be considered, challenging the generalisability of Siffre’s findings.

Who challenged the assumption that light is the dominant exogenous zeitgeber?

Buhr et al

Suggested that body temperature also plays a key role in regulating the sleep wake cycle as it controls the activity of cells, challenging the assumption that light is the dominant exogenous zeitgeber, supporting a more holistic view that multiple biological and environmental factors interact.

Evaluate circadian rhythms

Strengths

Practical application

Point: Research has led to real life application, improving health and productivity.

Evidence: Studies of shift work show that circadian disruption can lead to reduced concentration.

Explain: Understanding circadian rhythms has helped design better work schedules e.g. starting at peak productivity times 9-11.

Link: Increases the usefulness of the research, highlighting its relevance.

Evaluate circadian rhythms

Limitation

Individual differences

Point: Research into circadian rhythms often assume a universal pattern.

Evidence: Duffy et al found that some people prefer to sleep and wake earlier whilst others are the oppposite.

Explain; Shows that people have different sleep wake preferences.

Link: Means that findings from studies life Siffre’s may not apply to everyone, limiting the generalisability.

Evaluate the circadian rhythm

Limitation

Deterministic

Point: Many studies rely on light as the main exogenous zeitgeber.

Evidence: Buhr et al demonstrated that body temperature can influence circadian rhythms by affecting cell activity.

Explain: Highlights that other factors e.g. temperature, genetics, lifestyle. Play a role

Link: A holistic approach, acknowledging multiple influences, proves a more accurate and realistic understanding of biological rhythms.

Evaluate Circadian rhythms

Limitation

Case studies

Point: Most research has been based on case studies - siffre

Evidence Siffre’s findings came from his own experienceS and he also found that his own rhythms varied across replications of his study.

Explanation: Small sample sizes and individual variability make it hard to draw general conclusions.

Link: Limits external validity and reduces confidence in applying findings broadly.