A2 level- The Circadian Rhythm

Biological rhythms: The Circadian Rhythm

A2 level revision notes

Biological rhythms are called cyclical changes in the way biological systems behave. They are controlled by: Internal biological clock (Endogenous pacemakers) and External cues from the environment (exogenous zeitgebers).  

• ·        Circadian Rhythms -the rhythms that last around 24-hours. They are physical, mental and behavioural changes, responding to light and environment in an organism’s environment. Circadian comes from the Latin ‘Circa’ (about) plus ‘dies’ (a day). The two well-known Circadian rhythms are the sleep-wake cycle and the body temperature cycle.

• ·        The sleep-wake cycle -controlled by the body’s circadian rhythms. One might think the reason you go to sleep is because of the changes in daylight. You feel sleepy when it gets dark and are awoken when sunlight comes through the curtain, or your sleep-wake cycle is based on your knowledge of what time of day it is. These are external cues called exogenous zeitgebers. However there is also an internal (endogenous) clock. This clock is free running i.e. works without external cues (exogenous zeitgebers). Sets a cycle of about 24-25 hours. In normal circumstances the internal clock does not work alone. There are external cues (daylight) which help adjust the internal clock to the environment in which you live. These studies show circadian rhythms persist despite isolation from natural light, which proves an existence of an endogenous clock. However this research also shows that external cues are importance as the clock was not perfectly accrual: varied from day to day.

• ·        Core body temperature -one if the best indicators of the circadian rhythms. Lowest at 4.30am (36c), highest at 6.00pm (38c). There is a slight trough just after lunch. Not just due to effect of having lunch- also occurs even when people don’t eat. In many countries, have an afternoon siesta (sleep in hot country) is related to this dip in body temperature. Temperature dip is a bi-daily rhythm, example of the ultradian rhythms. 

• ·        Hormones – Hormone production follow a circadian rhythm. Cortisol lowest around midnight and peaks at 6.00am. Cortisol produced when stressed but also related to making us alert when we wake up. Explains if we are awoken at 4.00am is it hard to think clearly. Because cort levels are not sufficiently high for alertness. Melatonin and growth hormones also have clear circadian rhythm, peaking at midnight. 

Evaluation

• ·        AO2 RESEARCH SUPPORT: FREE-RUNNING RHYTHM- Michael Siffre, French cave explorer, conducted a series of studies to find evidence for free-running circadian rhythms. He has spent long periods of time living underground in order to study his own biological rhythms. Whilst he was underground he had no external cue’s to guide his rhythms. (Clock, daylight, radio). He woke, ate, and slept whenever he felt like it. The only thing influencing his behaviour was his internal ‘clock’. (1^st occasion) 61 days in the Southern Alps in 1962. He resurfaced on September 17^th thinking the date was the 20^th of August! (2^nd occasion) He spent 6 months in a Texan cave in 1975. His natural circadian rhythm settled down to just over 24 hours but would sometimes change to as much as 48 hours. In 1999 he was interested in the effects of ageing on biological rhythms (60 years old). He found that his internal clock was slower compared to when he was a young man. He also found his sleep patterns had changed. His study was supported by other studies: Aschoff and Wever 1976, Place participants in underground WW2 bunker. No environmental and social time cues. Found more participants displayed circadian rhythms between 24-25 hours. Some were as long as 29. This shows the cycle operates in the absence of external cues and natural free running cycle is about 24-25 hours.
• ·        AO2 RESEARCH SUPPORT: IMPORTANCE OF EXTERNAL CUES- Folkard 1985 conducted an experiment to see if external cues could be used to override the internal clock. 12 participants lived in a cave for 3 weeks, no natural light and other time cues. They agreed to go to bed when clock indicated 11.45pm and get up when indicated 7.45am. Initially the clock ran normally but gradually quickened until it was indicating passing of 24 hours when only 22 had passed. Beginning: circadian cycle matched the clock. When quickened their rhythm ceased to match the clock and continued to flow a 24-hour cycle rather than the 22 imposed by the experiment. (Apart from 1). Conclusion: Circadian rhythm can only be guided to a limit extent by external cues. As soon as experiment was over participants took days to resynchronise their cycles to the available external time cues (clocks, daylight). Showing influence of external cues.
• ·        AO2 SLEEP WAKE CYCLE: METHODOLOGY- Early research suffered from a flaw, when estimating the free running cycle. Participants were isolated from variables such as clocks and daylight. However they were not isolated from artificial light. It was thought that dim light (contrast to daylight) would not affect the circadian rhythm. Recent research shows this may not be true. Czeisler et al 1999 altered participant circadian rhythms down to 22hours and up to 28 hours just using dim lighting.
• ·        AO2 SLEEP WAKE CYCLE: INDIVIDUAL DIFFERENCES- 1) Cycle length. Czeisler et al 1999 found circadian cycles in different people can vary from 13-65 hours. 2) Cycle onset. Individuals appear to be innately different in terms of when their circadian cycles reaches their peak. Duffy et al 2000 found morning people prefer to rise early and go to bed early (6am-10pm). Evening people wake and go to bed later (10am-1am).