7. circadian rhythms

biological rhythms

distinct patterns of changes in body activity that conform to cylical time periods

they are influenced by internal body clocks (endogenous pacemakers) as well as external changes to the environment (exogenous zeitgebers)

circadian rhythms

biological rhythms that are subject to a 24 hour cycle

they regulate a number of body processes such as the sleep/wake cycle and changes in core body temperature

what governs the sleep/wake cycle

• daylight (EZ) as we feel drowsy when its night time and alert during the day

• a biological clock known as the superchiasmatic nucleus (SCN) (EP) which lies above the optic chiasm which provides information from the eye about light - light (EZ) can reset the SCN

siffre’s cave study procedure

• spend several extended periods underground to study the effects on his own biological rhythms

• he was deprived of exposure to natural light and sound but had access to adequate food and drink

• did it for two months in caves in the southern alps, and six months in a texan cave

• conducted daily experiments: took blood pressure, memory and physical tests

siffre’s findings

• in each case his free running biological rhythm settled down to one that was just beyond the usual 24 hours (around 25)

• continued to fall asleep and wake up on a regular schedule

siffre’s conclusion

• thought astronauts could manage their biological rhythm without EZs; however they would need companionship as the isolation would not be manageable

• concluded that time is not something humans could work with and understand without any external environmental cues

strengths of study

• provided a lot of quantitative and qualitative data

• study was done over a long time (6 months) allowing him to investigate his sleep/wake cycle and show how irregular it became

weaknesses of study

• when he woke up, strong lights were put on and when he went to sleep turned off - they could have acted as an external cue which affects its internal validity

• he was the only participant so it is hard to generalise the findings, such as he did to astronauts

over research on circadian rhythms

• aschoff and wever

• folkard et al

aschoff and wever procedure

• participants spent four weeks in a WW2 bunker

• deprived of natural light

findings

• all but one (whose sleep/wake cycle extended to 29 hours) displayed a circadian rhythm between 24 and 25 hours

• both siffre’s experience and this suggests the natural sleep/wake cycle may be slightly longer than 24 hours but that it is entrained by EZs associated with our 24 hour day (eg daylight hours, typical mealtimes)

folkard procedure

• 12 particpants

• agreed to live in a dark cave for 3 weeks

• retired to bed when the clock said 1145pm and rising when it said 0745am

• over the course of the study the researchers gradually sped up the clock (unbeknown to participants) so an apparent 24 hour day eventually lasted 22 hours

findings

• influence of EZs on internal biological clock should not be overstated

• only one participants could comfortably adjust to the new regime

• suggests the existance of a strong free running circadian rhythm that cannot easily be overridden by EZs

evaluation

• shift work (& 2x COUNTERPOINT)

• medical treatment

• individual differences

shift work

• provides an understanding of the adverse consequences that occur when they are disrupted (desynchonisation)

• research has found night workers engaged in shift work experience reduced concentration at 6am and that there is a relationship between shift work and poor health - 3x more likely to develop heart disease than those with typical work patterns

• has real world economic implications in managing health and safety, and how to best manage worker productivity (eg adjusting work schedules to reduce risks)

counterpoint

• studies investigating the effects of shift work tend to use correlational methods

• means its difficult to establish whether desynchronisation of the sleep/wake cycle is actually a cause of negative effects

counterpoint

• other variables may explain negative effects

• research has concluded that high divorce rates in shift workers may be due to the strain of deprived sleep and other influences such as missing out on important family events - hard to isolate the biological impact from social/lifestyle factors as correlation does not equal causation

• suggests caution when attributing all shift work effects solely to circadian rhythm disruption

medical treatment

• circadian research improves drug effectiveness through timing medication to enhance treatment

• aspirin as a treatment for heart attacks is most effective if taken last thing at night as they are most likely to occur early in the morning - it reduces blood platelet activity reducing heart attack risk

• practical use in chronotherapeutics where treatment is tailored to biological timing for maximum benefit

individual differences

• generalisations are difficult due to individual variation

• studies are small scale with just one participant in siffres, and sleep/wake cycles vary widely from person to person - research shows from 13 to 65 hours. siffre also found that in a later study his own sleep/wake cycle had slowed since he was younger

• there is no one size fits all biological rhythm so findings may not apply broadly, underminig external validity of circadian rhythm research and its real world applications