Despite the above criticism, there is support by other studies for Siffre’s case study. For example, Aschoff and Wever (1976) placed participants in an underground World War II bunker in the absence of social and environmental cues. They found that most people displayed circadian rhythms between 24 and 25 hours, though some rhythms were as long as 29 hours. These studies show that circadian rhythms persist despite isolation from natural light, which demonstrates the existence of an endogenous ‘clock’. However, this research also shows that external cues are important because the clock was not perfectly accurate: it varied from day to day. When looking at studies involving the sleep wake cycle, research methodology needs to be accounted for. In all studies, participants were isolated from variables that might affect their circadian rhythms such as clocks, radios and daylight, in. However, they were not isolated from artificial light because it was thought that dim light, in contrast to daylight, would not affect the circadian rhythm. Recent research by Czeisler et al (1999) suggests that this may not be true as they altered participants’ circadian rhythms down to 22 hours and up to 28 hours just using dim lighting.
Folkard et al (1985) conducted an experiment to see if external cues could be used to override the internal clock. A group of 12 people lived in a cave for three weeks, isolated from natural light and other time cues. These volunteers agreed to go to bed at 11.45 pm and to get up when it indicated 7.45 am. Initially the clock ran normally, but gradually they quickened the clock until it was indicating the passing of 24 hours when actually only 22 hours had passed. At the beginning, the volunteers circadian rhythms matched the clock, but, as it quickened, their rhythm no longer matched the clock and continued to follow a 24-hour cycle rather than the 22-hour cycle imposed by the experiment (except for one participant who did adapt to the 22-hour cycle). Overall, this suggests that the circadian rhythm can only be guided to a limited extent by external cues. However, there are exceptions as individual differences need to be taken into account. One important type of individual difference is the cycle length: research has found that circadian cycles in different people can vary from 13 to 65 hours (Czeisler et al, 1999). The second type of individual difference relates to cycle onset – individuals appear to be innately different in terms of when their circadian rhythms reach their peak. For example, Duffy et al (2000) found that morning people prefer to rise early and go to bed early (about 6am and 10pm), whereas evening people prefer to wake and go to bed later (10am and 1am).
Hormone production also follows a circadian rhythm. Cortisol is at it’s lowest around midnight and peaks around 6am. Cortisol is a hormone produced when we are stressed but is also related to making us alert when we wake up, and can explain why, if we awaken at 4am, it is hard to think clearly. This is because cortisol levels are not sufficiently high for alertness. Melatonin (which induces sleep) and growth hormone are two other hormones that have a clear circadian rhythm, both peaking at around midnight. The studies mentioned above are typical of the biological approach to understanding behaviour: they propose that human behaviour can be explained in terms of structures in the brain and in terms of hormonal activity. However, human behaviour is often more complex than this because people can override biologically determined behaviours by making choices about what they do which relates to the debate of freewill against determinism. On the other hand, sometimes it may not be possible to override biological factors and biological rhythms may be a case in point.
Core body temperature is one of the best indicators of the circadian rhythm. It is lowest at about 4.30am (about 36˚) and highest at around 6pm (about 38˚). There is a slight trough just after lunch which is not just due to the effects of having had lunch as the dip occurs even when people have not eaten. The circadian variation in core body temperature has been linked to cognitive abilities. For example, Folkard et al (1977) looked at the learning ability of 12-13 year old children who had stories read to them at either 9am or 3pm. After one week, the afternoon group (higher core body temperature) showed both superior recall and comprehension, retaining about 8% more meaningful material. This suggests that long-term recall is best when body temperature is highest. There is evidence that temperature changes do actually cause the changes in cognitive performance. Giesbrecht et al (1993) lowered body temperature (by placing participants in cold water) and found that cognitive performance was worse on some tasks. However, other research has found that the link is spurious. For example, Hord and Thompson (1983) tested cognitive performance in a field rather than lab situation and didn’t find any correlation between core temperature and cognitive performance. It may be that the higher core body temperature leads to increase physiological arousal and this leads to improved cognitive performance (Wright et al, 2002)
Circadian rhythms are part of the daily lives of humans. They cue our levels of alertness, our need for sleep, and our time of waking. Research shows that light, hormones, food, and a variety of other factors are important in determining circadian rhythms. Our fairly consistent sleep pattern suggests they are innate and not learnt meaning there is an internal or endogenous mechanism – the biological clock. However, this can be overridden by psychological factors, such as anxiety.
