Biological rhythms - Circadian Rhythms

Question 1

what is a circadian rhythm

Answer 1

Circadian rhythms last about 24 hours and are set/reset by environmental light levels.

Question 2

Three examples of circadian rhythms in the body:

Answer 2

Sleep/wake cycle
Temperature cycle
Hormone production

Question 3

Our body clock is regulated by an internal system including such factors as….

Answer 3

release of hormones like melatonin (from the pineal gland), metabolic rate and body temperature.

Question 4

what is an endogenous pacemaker

Answer 4

Internal body ‘clocks’ that regulate biological rhythms.

Question 5

what are Exogenous zeitgebers

Answer 5

Environmental cues, such as light, that help to regulate the biological clock in an organism.

Question 6

what is the suprachiasmatic nucleus (SCN)

Answer 6

the master circadian pacemaker, found in the hypothalamus.

Question 7

what is the primary input to the SCN

Answer 7

Light provides the primary input to this system, setting the body clock to the correct time each day in a process called photoentrainment.
Entrainment: an adjustment of the body clock in line with the environment

Question 8

The internal circadian ‘clock’ is described as ‘free running

Answer 8

‘Free-running’ means that the sleep-wake cycle will maintain a cycle of about 24-25 hours, even in the absence of external cues.

Question 9

Factors that could significantly impact the sleep/wake cycle include

Answer 9

jet travel and shift work, as they cause the biological clock e.g. the SCN (and the physiological systems that are dependent on this e.g. the sleep/wake cycle) to become completely out of sync with the outside world.

Question 10

Case Study: Michel Siffre (1975)
procedure

Answer 10

Siffre (a French cave explorer) spent 6 months in a cave with no natural light or cues as to the day or time e.g. clocks or radio.
He simply woke, ate and slept when he felt it was appropriate to do so and his internal body clock was allowed to free-run.

Question 11

Case Study: Michel Siffre (1975)
findings

Answer 11

Key findings: It settled into a sleep/wake cycle of 25-30 hours.
He lost track of how many days he had been in the cave, believing it to be one month less than he had actually stayed in.

Question 12

Case Study: Michel Siffre (1975)
conclusions

Answer 12

Conclusions: This suggests that circadian rhythms persist despite isolation from natural light, which demonstrates the existence of an endogenous clock i.e. his SCN continued to regulate his sleep/wake cycle.
However, it also shows that external cues are important in terms of keeping the sleep/wake cycle in sync with the outside world, because his clock was not exactly 24 hours.

Question 13

Evaluation of Michel Siffre’s (1975) research

Answer 13

:( The study of Michel Siffre is a case study and therefore has unique features. His body’s behaviour may not be typical of all people (lacks population validity).

:( Living in a cave may have particular effects due to, for example, the fact that it was cold could have affected certain physiological processes.

:(Siffre was not isolated from artificial light. On waking Siffre switched on lights, which is likely to have artificially re-set his body clock.

:)Siffre’s study was also an experiment - he controlled key variables (exogenous zeitgebers) to observe the effects on his sleep-wake cycle, demonstrating causal relationships.

Question 14

Aschoff and Wever (1967) procedure and findings

Answer 14

Participants were placed in an underground WWII bunker, in the absence of any environmental and social time cues.
They found that most participants displayed circadian rhythms between 24 and 25 hours in length, although some rhythms were as long as 29 hours.
This shows that the sleep/wake cycle operates in the absence of external cues and that the natural free-running cycle is about 24-25 hours.

Question 15

Folkard et al. (1985)
Procedure

Answer 15

Conducted an experiment to see if external cues could be used to override the internal clock.
A group of 12 people (18-21 yrs) lived in a temporal isolation unit for three weeks, isolated from natural light and other time cues, except a clock.
These volunteers agreed to go to bed when the clock indicated 11.45pm and to get up when it indicated 7.45am.
Initially, for the first 4 days, the clock ran normally, but gradually they quickened the clock until it was indicating the passing of 24 hours when actually only 22 hours had passed.
At the beginning the volunteers’ circadian cycle matched the clock but as it quickened their rhythm ceased to match the clock and continued to follow a 24-hour cycle rather than the 22-hour cycle imposed by the experiment (except for one participant who did adapt to the 22-hour cycle).
Overall, this suggests that the circadian rhythm can only be guided to a limited extent by external cues.

Question 16

Evaluation of research into the sleep/wake cycle
Research methodology

Answer 16

In most studies, although participants were isolated from variables that might affect their circadian rhythms, such as clocks, radios and daylight, they were not isolated from artificial light because it was thought that dim artificial light, in contrast to daylight, would not affect their circadian rhythms…
Therefore, artificial light could have confounded research findings when estimating the ‘free running’ nature of the human circadian rhythm.
Czeisler et al. (1999) found evidence to demonstrate the influence of artificial light on the sleep/wake cycle:
They altered participants’ circadian rhythms down to 22 hours and up to 28 hours by using dim artificial lighting alone.

Question 17

:(Individual differences in the sleep/wake cycle

Answer 17

Cycle length: research has found that circadian cycles can vary anywhere from 13 to 65 hours (Czeisler et al., 1999).

Cycle onset: individuals appear to be innately different in terms of when their circadian rhythms peak….

Question 18

Chronotherapeutics
This is the treatment of an illness or disorder by administering a drug/treatment at a time of day believed to be in harmony with the body’s natural rhythms i.e. at a time when it will be more effective.

Answer 18

Research has shown that chemotherapy is more effective in treating cancer when administered at specific times in the day. It was found that the timing of medication damaged 5x fewer healthy cells (if administered when healthy cells are least active) and was twice as effective at killing the cancerous cells.
Also, you are at greatest risk of having a heart attack in the early hours of the morning. Aspirin would therefore be most effective if taken at around 11pm, as it takes between 2-4 hours to peak in the blood stream.
This shows a clear practical implication for research on biological rhythms

Question 19

Hormone Production

Answer 19

Hormone release follows a circadian rhythm.
For example, the production and release of melatonin from the pineal gland in the brain follows a circadian rhythm, with levels peaking during the hours of darkness and decreasing in response to light in the morning.
Melatonin encourages sleep

Question 20

Cortisol levels..

Answer 20

..have also been found to fluctuate throughout the day and increase about an hour before a person wakes up, helping with alertness.