Describe and evaluate research into circadian rhythms (24 marks)

Describe and evaluate research into circadian rhythms (8+16)

Research into circadian rhythms has assessed the effects of isolating participants from external time cues (exogenous zeitgebers) on our 24 hour cycles, and suggesting that an internal body clock (endogenous pacemaker) creates a free running sleep/wake cycle rhythm of 25 hours.
For example Siffre (1975) reported a case study of his own experiences in an underground cave for 2 months. Without any exogenous zeitgebers such as light or cues to guide him, his sleep/wake cycle generally adjusted to a 25 hour cycle, though sometimes changing dramatically up to 48 hours.
                Similarly, Aschoff and Weaver (1976) also designed a temporal isolation study by placing participants in an underground ww2 bunker without an environmental or time cues. They found that the free running cycle persisted with a sleep wake cycle of 25 hours, sometimes increasing to 29.

A weakness into research of circadian rhythms is that it can be argued they lack internal validity. Although they removed natural light sources, artificial light from torches and cameras have been shown to reset the endogenous rhythms, thus interfering as an extraneous variable.
                This is exemplified through Czeisler et al (1999) who altered participants’ circadian rhythms down to 22 hours and up to 28 only using dim lighting to alter his release of melatonin from the pineal gland. As a result, the findings do not reflect true isolation and have lower internal validity, so his rhythm may not have adjusted so well if he had been truly isolated.
Similarly, Siffre’s study has been criticised for being a case study reflecting Siffre’s personal individual differences which cannot be generalised to the wider population (e.g. his motivation to be isolated and his bodily history of cave exploring).

Alternatively, the use of animals in studies investigating the function of the endogenous pacemaker can be praised for its scientific approach, yet criticised on ethical grounds. Morgan (1995) investigated the role of endogenous pacemakers, in particular the SCN, in the circadian rhythms of hamsters. Morgan removed the SCN in hamsters…