7) Biological Rhythms: Circadian Rhythms

What two things are all biological rhythms governed by?

• Endogenous pacemakers (internal structures)

  • e.g: suprachiasmatic nucleus

• Exogenous zeitgebers (external structures)

  • e.g: light

Define the process of entrainment?

Exogenous zeitgebers reset our body clock every 24 hours.

Explain what is meant by a biological rhythm?

Distinct patterns of changes in body activity that conform to cyclical time periods.

• Influenced by internal body clocks and external changes to the environment.

Explain what is meant by a circadian rhythm, and give one example?

A type of biological rhythm subject to a 24 hour cycle which regulates a number of body processes.

• e.g: the sleep/wake cycle

• e.g: changes in core body temperature

Describe the sleep wake cycle?

The sleep-wake cycle is an example of a circadian rhythm which dictates when human and animals should be asleep and awake.

• Light provides the primary input to this system, acting as the external cue for sleeping or waking.

  • Light is first detected by the eye, which then sends messages concerning the level of brightness to the suprachiasmatic nuclei (SCN).

• The SCN then uses this information to coordinate the activity of the entire circadian system.

  • Dependant on light, the SCN either inhibits or stimulates the production of melatonin (sleepy hormone) from the pineal gland.

    • E.G: Light induced activation of the SCN prevents the production of melatonin (sleepy hormone) by the pineal gland. ( this is WAKING UP)

Describe what is meant by homeostatic control in relation to the sleep-wake cycle?

In the absence of light, a circadian rhythm is considered as ‘free running’ or under homeostatic control.

• When an individual has been awake for a long time, energy consumption makes us sleep, regardless of day or night.

• As sleep pressure increases, so does our need for sleep.

Name a researcher who investigated homeostatic control?

Michel Siffre

Describe the purpose, procedure and findings from Siffre’s Cave Study?

(CAN BE AN EVALUATION)

(1962)

SIFFRE:

He spent several extended periods underground to study the effects on his own biological rhythms.

• He was completely deprived of exposure to natural light and sound, but had access to adequate food and drink for two months, beginning in July.

When he resurfaced, he believed it was August, rather than mid-September.

• This is because his free running circadian rhythm had changed from 24 hours to 24.7 hours.

  • This shows us homeostatic control can relatively effectively maintain your body clock in the absence of natural light.

Describe body temperature as a circadian rhythm?

Human body temperature is at its lowest in the early hours of the morning (36*C at 4:30am) and its highest in the early evening (38*C at 6pm).

• Sleep typically occurs when the core temperature starts to drop, and the body temperature starts to rise towards the end of a sleep cycle- promoting feelings of alertness first thing in the morning.

Evaluate the strengths of circadian rhythms?

STRENGTH: SHIFT WORK

Research can increase our understanding of the adverse effects of disrupting circadian rhythms.

• Shift workers are more likely to develop heart disease and cancer.

BOIVIN ET AL

• Night shift workers experience a dip in concentration at 6am.

  • So increases our understanding of accidents at work and other situations where this may be a mitigating factor involved.

KNUTSSON:

• Reports higher rates of divorce for shift workers.

So there are real world applications of the importance of following our natural circadian rhythms, for our health and wellbeing.

STRENGTH: MEDICAL TREATMENT

The rise and fall of a number of basic rhythm processes controlled by circadian rhythms has led to the development of chronotherapeutics.

• We now know that certain medical conditions are more prevalent at certain times.

  • (e.g: chronic pain worst at night)

• Therefore, we can use chronotherapeutics to allow a drug to become active at a certain time of the day that aligns with our circadian rhythms so it is most effective.

  • e.g: aspirin is most effective when taken late at night as heart attacks often occur first thing in the morning.

Evaluate limitations of circadian rhythms?

LIMITATION: COUNTERPOINT TO SHIFT WORK
High divorce rates could be explained by a third factor such as regularly missing out on family events due to the strain of sleep deprivation.

• Therefore, more research must be conducted to determine the cause and effect between desynchronisation of circadian rhythms and any potential adverse effects.

LIMITATION: INDIVIDUAL DIFFERENCES

Many studies on this topic involve small sample sizes, this could cause issues for generalising to a large population.

• Rhythms also appear to vary.

CZEISLER ET AL:

• Circadian rhythms can vary from 13-65 hours.

DUFFY ET AL: (LARKS VS OWLS)

• Some people have a natural preference for going to bed earlier and rise earlier (larks).

• While, others have a preference for going to bed later and rising later (owls).

SIFFRE:

• Rhythms slow down with age.

Therefore, it is hard to make generalisations as there is great variation between people.