Circadian rhythms

What are biological rhythms?

Cyclical changes in the way that biological systems behave.

What are the different types of biological rhythms?

Circadian rhythms

Ultradian rhythms

Infradian rhythms

Why do these systems exist?

Because the environment organisms live in has cyclic changes (eg day/night, winter/summer).

Circadian rhythms definition

A cycle lasting around a day - 24 hours

Often referred to as the body clock.

Advantage of having circadian rhythms.

They optimise an organ’s physiology and behaviour to best meet the varying demands of the day/night cycle.

How are our circadian rhythms driven?

By our body clocks - which are found in all cells of the body -, the master circadian pacemaker and the suprachiasmatic nuclei (SCN) - found in the hypothalamus.

What constantly happens to the master circadian pacemaker? Why?

The pacemaker is constantly reset, so that our bodies are in synchrony with the outside world.

What is the primary input to this system?

Light.

What does light do in this system?

It sets the body clock to the correct time in a process called photoentrainment.

How does the SCN receive messages about environmental light levels in mammals?

Light-sensitive cells within the eye act as brightness detectors, sending messages about environmental light levels directly to the SCN.

What does the SCN then do with this information?

It uses it to coordinate the activity of the entire circadian system.

Which cycle is subject to this entrainment process?

The sleep-wake cycle

What is the sleep-wake cycle?

Alternating states of sleep and waking that are dependable on the 24-hour circadian cycle.

The sleep wake cycle: What are the external signals that determine when we feel the need to sleep and when to wake up.

Light and darkness.

What happens with the circadian rhythm at different times of day?

It dips and rises.

So our strongest sleep drive usually occur between 2-4am and between 1-3pm (the ‘post-lunch dip’)

How is the intensity of sleepiness we experience during these dips determined?

The sleepiness is less intense if we’ve had sufficient sleep, and more intense when we’re sleep deprived.

Sleepiness and wakefulness are not determined by the circadian rhythm alone. What else is it determined by?

They are also under homeostatic control.

When we’ve been awake for a long period of time, homeostasis tells us that we need sleep more because of the amount of energy being used up during wakefulness.

This homeostatic drive for sleep gradually increases throughout the day, reaching its maximum in the late evening.

So how do the circadian system and the homeostasic system work together to keep us awake?

The circadian system keeps us awake in daylight, prompting us to go to sleep as it becomes dark.

The homeostatic system makes us sleepier as time goes on throughout the waking period, regardless of whether it’s night or day.

The circadian system is described as ‘free running’. What does this mean?

It will maintain a cycle of about 24 hours, even in the absence of external cues.

How can the circadian system become out of balance?

As a result of major alterations in sleep and wake schedules (eg jet lag).

What are the two other circadian rhythms?

Core body temperature and hormone production.

Explain the circadian rhythm of core temperature.

It’s at its lowest temperature at about 4:30 am and its highest temperature at about 6 pm.

Sleep tends to occur when the core temperature begins to drop, and body temperature starts to rise during the last hours of sleep - promoting a feeling of alertness in the morning.

Explain the circadian rhythm of hormone production.

Example:

The production and release of melatonin from the pineal gland in the brain follows a circadian rhythm, with peak levels occurring during the hours of darkness.

By activating chemical receptors in the brain, melatonin encourages feelings of sleep. When it’s dark, more melatonin is produced. When it’s light, the production of melatonin drops as the person wakes.

CASE STUDY: what is the study showing evidence for a ‘free-running’ circadian rhythm?

Siffre’s 1962 cave study.

What was the aim of the study?

For Siffre to study his own circadian rhythms.

What was the procedure of the study?

Siffre spent 61 days living underground in the Alps.

He had no external cues to guide his rhythms - no daylight, clocks or radio.

The only thing influencing his behaviour was his internal body clock (‘free running circadian rhythms’).

What were the results of the study?

Siffre resurfaced on 17 September believing the date was 20 August!

His circadian rhythms had kept to slightly over 24 hours.

So what were the conclusions of the study?

Circadian rhythms are free running, but they aren’t perfect.

Positive eval

Research support for the importance of light - Hughes (1977) found that the cortisol release of patients in Antarctica shifted after 3 months of darkness. This shows that extreme daylight variation affects circadian rhythms, highlighting the importance of light.

RWA - circadian rhythms can be used in chronotherapeutics - the study of how timing affects drug treatments. Drugs can be timed with circadian rhythms (eg heart attack risk in the morning).

Negative eval

Siffre was exposed to artificial light in his cave study. Research from Czeisler (1999) found that cycle length can vary from 22 to 28 hours with light exposure. So Siffre’s study may lack validity.

Buhr (2010) believes that it is actually temperature that controls our body clock, rather than light. Buhr found that temperature fluctuations influence timing of cells, tissues and organs. So temperature may be more important than light.