biological rhythms and circadian

what is a biological rhythm

• Refer to change in body processes or behaviour that repeats regularly in a cycle i.e. going to sleep when its dark and waking up when its light

• Biological rhythms are influenced by internal body clocks (endogenous pacemakers) and external changes in the environment (exogenous zeitgebers)

• All living organisms are subject to biological rhythms and these exert an important influence on the way in which body systems behave

• Some rhythms occur many times during the day (ultradian rhythms)

• Some take longer than a day to complete (infradian rhythms)

• Some take much longer (circannual rhythms)

• And there are also circadian rhythms which last for around 24 hours

what is a circadian rhythm

• One biological rhythm is the 24-hour circadian rhythm, which is reset by light

• Circadian is from the Latin 'circa' which means 'about' and 'dian' which means 'day'

• Examples are

  • Sleep-wake cycle

  • Body temperature

  • Hormone production

• Biological rhythms are regulated by endogenous pacemakers and exogenous zeitgebers

  • Exogenous zeitgebers = external cues such as light, social activities and cues etc.

  • Endogenous pacemakers = internal body clocks that regulate many biological rhythms such as hormones (melatonin) and SCN in sleep/wake cycle

  • Both interact with one another to control and fine-tune biological rhythms and therefore it is necessary to consider these concepts together

• The fact that we feel tired at night and alert during the day is due to the effect of daylight on the sleep-wake cycle

  • Also governed by the SCN which can be reset by light

strengths of biological rhythms

Michel Siffre (1975)

• Studied his own circadian rhythms while living underground for long periods of time without any external cues to guide his rhythms (no daylight, clocks, sounds or radio)

  • Had access to food and drink

  • HE HAD ARTIFICIAL LIGHT

• Only thing influencing his behaviour to sleep, eat etc was his internal body clock ('free running circadian rhythm)

• When he emerged from his first underground stay lasting 61 days, he believed the date to be a month earlier than it was

• Additionally, his 24-hour sleep-wake cycle was increased making him believe one day was longer than it was and leading to his thinking that fewer days had passed

  • This was because his 'free running' rhythm settled down to one that was just beyond 24 hours though he continued to wake and sleep on a regular schedule

 

• Suggests that his 24-hour sleep-wake cycle was increased by the lack of external cues (exogenous zeitgebers) making him believe one day was longer than it was

• This shows that the absence of external cues e.g. daylight, significantly alter circadian rhythms

• Siffre's 'free running' cycle was brought back into line when he was reintroduced to natural day light showing the interaction of internal and external factors in regulating the sleep-wake cycle

 

Aschoff & Weber (1976)

• Studied participants living in a WW2 bunker for 4 weeks

• 12 people

• The bunker had no windows and only artificial light, which the ppts were free to turn on and off as they pleased

• They found that the ppts settled into a longer sleep/wake cycle of between 25-27 hours (except for one anomaly who had 29 hours)

• Suggest that humans use natural light (exogenous zeitgebers) to regulate the sleep-wake cycle and without light this cycle is disrupted

 

• Both Siffre and this study suggest that natural sleep/wake cycle is longer than 24 hours but is entrained by zeitgebers associated with our 24-hour day

limitations of biological rhythms

Counterpoint Folkard et al (1985) 

• Studied 12 people who lived in a dark cave for 3 weeks going to bed when the clock said 11:45 and rise when it said 7:45

• The researchers speeded up the clock (ppts unaware) so that a 24-hr day only lasted 22 hours

• Only one of the ppts was able to comfortably adjust to this new regime

 

• Suggests that strong free-running circadian rhythms cannot easily be overridden by exogenous zeitgebers (in this case the sped-up clock)

strengths of circadian rhythms

Strength - Understanding the adverse effects of disruption of biological rhythms (desynchronisation)

• Night workers engaged in shift work experience a period of reduced concentration around 6am in the morning (a circadian trough) meaning mistakes and accidents are more likely (Boivin et al 1996)

• Research has also pointed to a relationship between shift work and poor health - shift workers are three times more likely to develop heart disease than people who work more typical work patterns (Knutsson 2003)

• Research into circadian rhythms provides an understanding of the adverse consequences of that occur when they are disrupted (desynchronisation)

• Shows that research into sleep/wake cycle may have real-world economic implications in terms of how best to manage worker productivity

  • HOWEVER, problems with this research - correlational - difficult to establish whether desynchronisation of sleep/wake cycle is the cause of the negative effects

  • May be that night-time jobs are more inherently stressful

  • Solomon 1993 concluded that high divorce rates in shift workers is due to strain of deprived sleep and missing out on family events so may not be the biological factors that create adverse consequences associated with shift work

 

Chronotherapy - real world application strength

• Research into circadian rhythms has improved medical treatments

• Chronotherapy is the concept of administering drugs or treatments on a schedule that corresponds to a person's daily biological clock

  • Based on fact that circadian rhythm controls number of body's processes such as heart rate, digestion, hormone levels and these change during the day

• The time a drug or treatment is given can affect its effectiveness in some cases

  • e.g. heart attacks are most likely to happen during the morning, aspirin is typically taken last thing at night as a treatment of heart attacks as it is slow releasing and reduces platelet activity

• Chronotherapy is a growing field of medicine that has the potential to improve patient outcomes by tailoring medication administration to individual circadian rhythms

weaknesses

Flaws in 'free running' studies - limitation

• Not isolated from artificial light although this could have affected their circadian rhythm

• Small sample sizes = difficult to generalise the findings especially when there were known individual differences in the natural sleep-wake cycle of the ppts

• Individual differences (e.g. larks and owls) in the sleep/wake cycle means that participants may not represent the wider population

 

Individual differences in sleep/wake cycle so generalisations are difficult to make

• One is cycle length; research has found that circadian cycles can vary significantly from individual to individual (Czeisler et al 1999)

  • 13 to 65 hours

• Other type of individual difference relates to cycle onset - individuals appear to be innately different in terms of when their circadian rhythms reach their peak

• Duffy et al (2001) found that 'morning people' prefer to rise and go to bed early (about 6am and 10pm) whereas 'evening people' prefer to wake and go to bed later (about 10am and 1am)

  • This demonstrates that there may be individual differences in circadian rhythms

• Siffre's own cycle slowed down as he got older so it is age-related too