Biological Rhythms

• One biological rhythm is the 24-hour circadian rhythm.
• The circadian rhythm is often known as the internal ‘body clock’.
• The sleep-wake cycle is an example of a circadian rhythm.
• Body temperature is a circadian rhythm. Human body temperature is at its lowest in the early hours of the morning and at its highest in the early evening.
• Circadian rhythms last about 24 hours (e.g. the sleep-wake cycle) and are controlled by an endogenous pacemaker.
• Siffre (1975) found that the absence of external cues significantly altered his circadian rhythm.
• There are differences between individuals when it comes to circadian rhythms.
• Biological rhythms are regulated by endogenous pacemakers, which are the body’s internal biological clocks.
• Biological rhythms are also determined by exogenous zeitgebers, which are external cues, including light, that help to regulate the internal biological clocks.
• Although endogenous pacemakers are internal biological clocks, they can be altered and affected by the environment.
• The most important endogenous pacemaker is the suprachiasmatic nucleus (SCN), which is closely linked to the pineal gland, both of which are influential in maintaining the circadian sleep/wake cycle.
• The suprachiasmatic nucleus (SCN), which lies in the hypothalamus, is the main endogenous pacemaker.
• The SCN sends signals to the pineal gland, which leads to an increase in the production of melatonin at night, helping to induce sleep.
• Exogenous zeitgebers can include social cues such as meal times and social activities, but the most important zeitgeber is light.
• Morgan (1955) bred hamsters so that they had circadian rhythms of 20 hours rather than 24.
• Exogenous is external, whereas endogenous is internal.
• Many studies demonstrate the significance of the SCN and how endogenous pacemakers are important for biological circadian rhythms.
• When Siffre returned from an underground stay with no clocks or light, he believed the date to be a month earlier than it was. This suggests that his 24-hour sleep-wake cycle was increased by the lack of external cues.
• Aschoff & Wever (1976) found that people placed in a WW2 bunker underground with the absence of any environmental/ social cues displayed circadian rhythms between 24-25 hours, with some as long as 29 hours.
• Infradian rhythms are biological cycles that last longer than 24 hours and can be weekly, monthly or even annually.
• An example of a monthly infradian rhythm is the female menstrual cycle.
• Reinberg (1967) examined a woman who spent three months in a cave with only a small lamp to provide light. It was noted that her menstrual cycle shortened from the usual 28 days to 25.7 days.
• Russell et al (1980) found that female menstrual cycles became synchronised with other females through odour exposure (external factors).
• A second example of an infradian rhythm is related to the seasons in seasonal affective disorder (depression caused by a lack of light).
• In seasonal affective disorder (SAD) the lack of light during the winter months results in a longer period of melatonin secretion, which has been linked to the depressive symptoms.
• Terman (1988) found that the rate of SAD is more common in Northern countries where the winter nights are longer.
• Ultradian rhythms last fewer than 24 hours and can be found in the pattern of human sleep.
• The problem with studying sleep cycles is the differences observed in people, which make investigating patterns difficult.
• When investigating sleep patterns, participants must be subjected to a specific level of control and be attached to monitors that measure such rhythms, making them lack ecological validity.
• Folkard et al (1985) studied a group of 12 people who agreed to live in a dark cave for 3 weeks.
• Folkard et al (1985) found that when they manipulated the times on clocks to appear to be faster, some participants adjusted their cycles to the new times.
• One practical application from research on biological rhythms is for people doing shift work.
• Shift workers working during the night have to change their biological rhythms to suit their lifestyle.
• Shift workers working at night cannot be influenced by exogenous zeitgebers like lack of light, otherwise they could not do their jobs.
• Boivin et al (1996) found night workers experienced a lack of concentration around 6am, making more mistakes.
• Knutsson (2003) highlighted the relationship between shift work and poor health.
• Solomon (1993) concluded high divorce rates in shift workers may be due to lack of sleep.
• Circadian rhythms can help increase the effectiveness of drug therapy, by understanding the rise and fall of biological processes throughout the day.
• Duffy et al (2001) suggest that some people have a preference for going to bed early and rising early (larks) compared to those who prefer the opposite (owls).
• Czeisler et al (1999) found individual differences in sleep/wake cycles varying from 13 to 65 hours.