BR: circadian rhythms

• Biological rhythms- exert important influence on how body systems behave. All are governed by 2 things-

  -body’s Internal biological ‘clocks’- endogenous zeitgebers.

  -external changes in environment- exogenous zeitgebers.

  Some rhythms occur many times during day (ultradian R), and can be much longer (circannual R), lasting around 24 hours (sleep/wake cycle, & core body temperature).
• The sleep/wake cycle- The fact we feel drowsy when its night-time and alert during the day demonstrates the effect of daylight (important exogenous zeitgeiber) on our sleep/wake cycle.

  However the sleep/wake cycle is also governed by an internal (endogenous) pacemaker -a biological clock called the suprachiasmatic nucleus (SCN). It lies just above the optic chiasm which provides information from the eye about light. Exogenous zeitgebers (light) can reset the SCN.
• Siffre's cave study- caveman spent extended periods underground to study effects on his own Biological Rhythms.

  Deprived of exposure to natural light and sound, but could access food and drink. He resurfaced in mid-September after 2 months in Southern Alps caves, believing it to be mid-August!

  A decade later he replicated it but for 6 months in a Texan cave.

  =Both times, his 'free-running' biological rhythm went just beyond usual 24 hrs (25hrs) though he did continue to fall asleep and wake up on a regular schedule.
• Aschoff & Wever- group of p's spent 4 weeks in a WWII bunker deprived of natural light. All but one p (whose S/WC extended to 29 hours) had a CR of 24-25 hours.

  This study and Siffre's experience suggests the 'natural sleep/wake cycle' may be slightly longer than 24 hours, but it's entrained by exogenous zeitgebers associated with our 24-hour day (-typical mealtimes)
• Other research 2- Despite this, we should not overestimate influence of exogenous zeitgebers on our internal biological clock. Folkard et al. studied a group of 12 people who agreed to live in a dark cave for 3 weeks, retiring to bed when the clock said 11.45 pm and rising when it said 7.45 am. Researchers gradually sped up the clock (unbeknown to p's) so apparent 24-hour day eventually lasted only 22 hours. Only one p was able to comfortably adjust to the new regime. =The existence of a strong free-running circadian rhythm that cannot easily be overridden by exogenous zeitgebers.
• +One strength of research into circadian rhythms is that it provides an understanding of the adverse consequences that occur when they're disrupted (desynchronisation). Night workers engaged in shift work experience a period of reduced concentration around 6am (circadian trough) meaning mistakes and accidents are more likely (Boivin et al.). Research also points to a relationship between shift work and poor health -they're 3x more likely to develop heart disease (Knutsson). This shows that research into S/W C may have real-world economic implications- ITO managing worker productivity.
• -However, studies investigating effects of shift work tend to use correlational methods. This means it is difficult to establish whether desynchronisation of the sleep/wake cycle is actually a cause of negative effects.There may be other factors. Solomon concluded that high divorce rates in shift workers might be due to the strain of deprived sleep and other influences such as missing out on important family events. This suggests that it may not be biological factors that create the adverse consequences associated with shift work.
• +Another strength of research into circadian rhythms is its been used to improve medical treatments. CR co-ordinate a number of body's basic processes (heart rate, digestion). These rise and fall during a day, leading to field of chronotherapeutics -how medical treatment can be administered in a way that corresponds to a person's BR. -Aspirin as a treatment for heart attacks is most effective if taken last thing at night- reduces blood platelet activity, reducing risk of heart attacks that occur early in morning. Bonnet et +. Thus, CR research can help increase effectiveness of drug treatments
• -One limitation of research into circadian rhythms is that generalisations are difficult to make. Studies are based on very small samples of p's. It seems S/W C may vary widely from P to P. Research by Czeisler et al. found individual differences in S/W C varying from 13 to 65 hours. Duffy et al. study revealed some people have a natural preference for going to bed early and rising early ('larks') whereas others prefer the opposite (owls). Even Siffre, later observed his own S/W C had slowed down. Thus it's difficult to use research data to discuss anything more than averages- meaningless.