biological rhythms: circadian rhythms

• endogenous pacemaker is a biological clock without the influence of external stimuli
• exogenous zeitgebers are external changes in the environment. For example, we feel tired at night and awake during the day
• circadian rhythms last for 24 hours for example the sleep/wake cycle
• circadian rhythms are governed by SCN which lies just above the optic chiasm which provides information from the eyes about light. Exogenous zeitgebers (like light) can reset the SCN
• Siffre spent long periods in dark caves to examine the effects of free running biological rhythms (2 months in 1962 and 6 months in 1970). In each case study, Siffre's free running circadian rhythm settled down to around 25 hours. Also he did have a regular sleep/wake cycle
• Aschoff and Wever study on circadian rhythms: a group of participants spent 4 weeks in a WWII bunker deprived of natural lights. All but one (whose sleep/wake cycle extended to 29 hours) displayed a circadian rhythm between 24 and 25 hours
• the studies on circadian rhythms show that the 'natural' sleep/wake cycle may be slightly longer than 24 hours but is entrained by exogenous zeitgebers associated with our 24 hour day like meal times and daylight hours
• Folkard et al. study on endogenous pacemakers and exogenous zeitgebers: they studied a group of 12 people who lived in a dark cave for 3 weeks, going to bed when the clock said 11:45 pm and waking up when it said 7:45 am. The researchers gradually speeded up the clock (participants didn't know this) to a 22 hour day. Only one participant comfortably adjusted to the new regime. This suggests the existence of a strong free running circadian rhythm not controlled by exogenous zeitgebers
• One strength of circadian rhythm research is application to shift work. Shift work creates desynchronisation of biological rhythms. Researchers found shift worker experience a lapse of concentration around 6am so accidents are more likely. Research also suggests a link between shift work and poor health, with shift workers 3 times more likely to develop heart disease (Knutsson). Thus, research into the sleep/wake cycle may have economic implications in terms of how best to manage shift work
• One strength of circadian rhythms is real world application to medical treatment. Circadian rhythms co-ordinate the body's basic processes like heart rate and hormone levels with implications for chronotheraputics. Aspirin reduces heart attacks, which are most likely in the morning. Researchers found taking aspirin is most effective last thing at night. This shows that circadian rhythm research can help increase the effectiveness of drug treatments
• One limitation of circadian rhythms is that generalisations are difficult to make. Studies of the sleep/wake cycle often use small groups of participants like Aschoff and Wever or even single individual like Siffre. Participants may not be representative of the elder population and this limits making meaningful generalisation. Siffre observed that his internal clock ticked much more slowly at 60 than when he was younger. This suggests that, even when the same person is involved, there are factors that may prevent general conclusions being drawn