circadian rhythms

biological rhythm

distinct patterns of changes in body activity that conform to cyclical time periods, influenced by internal internal body clocks (endogenous pacemakers) and external changes in the environment (exogenous zeitgebers)

what are biological rhythms governed by?

all living organisms have biological rhythms that control the body’s systems, they’re governed by 2 things:

1. internal body ‘clocks’- endogenous pacemakers

2. external changes in the environment- exogenous zeitgebers

types of biological rhythms

1. ultradian rhythms- occur many times during the day

2. infradian rhythms- take longer than a day to complete

3. circadian rhythms- last around 24 hours eg sleep/wake cycle, core body temperature

circadian rhythms

biological rhythms, subject to a 24hr cycle, which regulate a number of body processes such as the sleep/wake cycle and changes in core body temperature

effect of core body temperature as a circadian rhythm

• varies by 2 degrees C over the day- 4am = lowest (36 degrees), 6pm = highest (38 degrees)

• the warmer we are, the better our cognitive performance

• Folkard et al. (1977)- children who were read to at 3pm showed better recall and comprehension after a week than children read the same stories at 9am

• Gupta (1991)- improved performance on IQ tests when assessed at 7pm compared to 2pm and 9am

effect of sleep/wake cycle as a circadian rhythm

• we naturally feel drowsy when its night time and more alert during the day, due to light which is an exogenous zeitgeber

• we also have an endogenous pacemaker controlling the cycle, the suprachiasmatic nucleus acts as a biological clock, lies just above the optic chasm, receiving information from the eye about light

• exogenous Zeitgebers (light) can reset the SCN

Siffre (1962) sleep/wake cycle experiment

wanted to explore what would happen to our sleep/wake cycle without the effect of exogenous zeitgebers, he entered a cave in the Southern Alps

• no exposure to natural light, spent 2 months in there but thought only 1 month had passed

• repeated this in the 70s for 6 months

• in the cave, Siffre noted he continued to fall asleep and wake on a regular schedule, however his ‘free running’ biological rhythm was 25 hours not 24

• suggests our internal pacemakers are pretty accurate, but need exogenous zeitgebers to keep them in clock

Aschoff + Wever (1976) sleep/wake cycle experiment

placed a group of participants in a WW2 bunker for 4 weeks, they were deprived of natural light

• 1 participant settled into a sleep/wake cycle of 29 hours, the rest showed a circadian rhythm of between 24 and 25 hours

• suggests the natural sleep/wake cycle is slightly longer than 24 hours but is entrained by the exogenous zeitgebers associated with society’s 24 hours day eg meal times, daylight hours, working hours

strength of circadian rhythm research- application to shift work

• shift work creates desynchronisation of biological rhythms, Bolvin et al. (1996) found shift workers experience a lapse of concentration around 6am (circadian trough) so accidents are more likely

• research also suggests a link between shift work and poor health, with shift workers 3x more likely to develop heart disease (Knutssch 2003)

• so research into the sleep/wake cycle may have economic implications in how best to manage shift work

counterpoint of circadian rhythms- application to shift work

• research into shift work is correlational, so the observed difficulties may not be fully due to desynchronisation, eg Solomon (1993) concluded high divorce rates in shift workers may be due to strain of deprived sleep and missing family events

• suggests there are other non biological factors influencing the link between desynchronisation and negative effects

strength of circadian rhythm research- real world application to medical treatment

• circadian rhythms co-ordinate the body’s basic processes, eg HR, hormone levels, with implications for chronotherapeutics (timing medication to maximise effects on the body)

• Bonten et al. (2015) found heart attacks are most likely in the morning, taking aspirin can reduce the risks by reducing blood platelet activity (thins it), found taking aspirin when risk is highest is most effective

• shows circadian rhythm research can help increase the effectiveness of drug treatments

limitation of circadian rhythm research- individual differences may make generalisation difficult

• sleep/wake research is based on small samples or single behaviours (Siffre), sleep/wake cycles vary eg Czeisler et al. (1999) found individual differences in cycle from 13 to 65 hours, Duffy et al. (2001) some people have a natural preference for going to bed and rising early (larks), some people prefer to sleep and get up late (owls)

• participants may not be representative of wider population so limits meaningful generalisations, Siffre found his internal clock ticked slower at 60 than young

• suggests even with the same person, there are factors preventing general conclusions

evaluation of circadian rhythm research- impact on the school/work day

• Wolfson + Carskadon (1998) suggests hormonal shifts in teenagers make sleep more difficult, teenagers are usually more tired at the start of the day so recommend starting school 2 hours later to fit the teenage sleep pattern (chronotype), adolescent sleep working group (2014) found starting school later improves academic performance and behaviour

• however, a later start is disruptive for parents and teachers and limits extracurricular activities, teenagers may just stay up later and be exhausted

• suggests changing the school day may not be practical even though its desirable