Lesson 11 - Circadian rhythms

Question 1

Frequency of circadian rhythms

Answer 1

Once every 24 hours/lasts about 24 hours

Question 2

Key features

Answer 2

• Controlled by the suprachiasmatic nuclei in the hypothalamus
• This acts as a pacemaker in order to reset the body so it is in synchrony with the outside world
• Natural light is the trigger for this reset and the process of re-acclimating the body using light is called photoentrainment

Question 3

Sleep-Wake Cycle

Answer 3

• Light and dark are the exogenous zeitgebers that determine the need to sleep.
• Peak sleepiness occurs between 2-4am and 1-3pm. If we are sleep sufficient, the urge is weak, if we are deprived, the urge is intense
• It is under homeostatic control

Question 4

Michel Siffre

Answer 4

• French cave explore Michel Siffre studied the human body clock.
• He spent periods of time underground with no indicator of time (light, clock, radio) so he had no exogenous zeitgebers
• He slept, woke, and ate when he felt like it
• He stayed for 61 days in the Southern Alps. He thought he emerged on the 20th of August, but in reality it was the 17th of September
• His body clocked maintained a biological rhythm of just over 24 hours. This is evidence that even without external cues, the body can maintain biological rhythms through homeostatic control

Question 5

Core body temperature

Answer 5

• Core body temperature is at its LOWEST (36˚c) at around 4:30am and at its HIGHEST (38˚c) at around 6pm
• The cooling of the body in the evening is associated with the release of melatonin and promotes the feeling of sleepiness. Lowest temperature being at 4:30am helps maintain sleep, preventing you from waking up.
• The body eventually increases its core body temperature, preparing you to wake up

Question 6

Evidence for core body temperature

Answer 6

• Folkard et al (1977) demonstrated that children who were read stories at 3pm had better recall after a week compared to children who heard the same story at 9am.
• Gupta (1991) found that participants performed better on an IQ test at 7pm compared to 2pm and 9pm. This is because there is a slight dip in body temperature between 2-4, which could account for why people are sleepy in the afternoon.

Question 7

Hormone production

Answer 7

The release of melatonin from the pineal gland is at its peak during hours of darkness. The production of melatonin stops when light is detected. Therefore it is produced once every 24 hours and is a circadian rhythm.

Question 8

Strengths of circadian rhythms

Answer 8

• Practical applications. Chronotherapeutics is the study of how timing of dosages affects treatment. By studying circadian rhythms, we know that the most vulnerable time for a heart attack is just after waking. So drugs that are taken at night can be designed to only be released at around 6am.
• Knutsson (2003) found that shift workers are 3 times more likely to develop heart disease. Research has found adverse consequences of disruption to the circadian rhythms. If you are on a night shift, you have low concentration levels at 6am, and this could cause an accident. Therefore employers can shift their hours so it is less busy at those times.

Question 9

Weaknesses of circadian rhythms

Answer 9

• Evidence for circadian rhythms are either case studies like Siffre or small sample sizes, so it is difficult to generalise this across the population. Siffre also found that his body clock ticked slower when he was 60 compared to when he was younger, so it also changes as you age.
• Early circadian research deprived participants of natural exogenous zeitgebers like daylight, but not artificial ones like a torch. Czeisler et al (1999) found that he could alter the internal clock to 22-28 hours by manipulating artificial lighting.