Exogenous zeitgebers

• Exogenous zeitgebers are any external stimuli that influence the biological rhythms of an organism
• They’re responsible for entraining the biological clock, and regulating circadian rhythms
• They act as cues for endogenous pacemakers, so that a person remains synchronised to the environment
• In the absence of these external cues, biological clocks continue to move in a free-running cycle. This can be shown in studies such as Siffre’s case study, where rhythms start to vary outside of the usual 24-hour cycle
• Light in the most important exogenous zeitgeber for most animals. Endogenous pacemakers like the SCN rely on the external cue of lights to coordinate their function
• Receptors in the SCN are sensitive to changes in light levels, and they use this information to synchronise the activity of the body’s organs and glands
• Social stimuli can also be argued to play a role as exogenous zeitgebers. Examples of these include mealtimes and social activities
• Aschoff et al (1971) found that individuals who aren’t exposed to exogenous zeitgebers in the form of natural light can compensate for this by regulating their biological rhythms in response to social cues
• Campbell and Murphy (1998) monitored the body temperature of 15 volunteers who slept in a laboratory. They woke the ps multiple times in the night to shine a beam of light onto the back of their knees, in attempt to disrupt their circadian rhythms. They found that the p’s circadian rhythms were disrupted by up to three hours
• Shih-Yu Lee et al (2013) investigated whether light therapy would help mothers of premature babies in intensive care. In these environments, the light is dim, and this disrupts the sleep/wake cycle of the mothers. To compensate for this, mothers were given bright light therapy over three weeks, and their sleep monitored. The women who received the light therapy reported an improvement in sleep quality compared to mothers who didn’t receive this treatment
• Miles et al (1977) suggests the role of exogenous zeitgebers is overstated. They conducted a case study of a blind man, who had a natural circadian rhythm of 24.9 hours. Despite exposure to social cues, his sleep/wake cycle couldnt be adjusted. He had to take sedatives at night and stimulants in the morning to keep pace with the 24 hour world. This contradicts the view that social cues are important zeitgebers in the absence of light, and may not be vital to the regulation of pacemakers in line with environmental cues
• Endogenous pacemakers provide stability in our sleep/wake cycles and exogenous zeitgebers provide the means to adjust this cycle when the environment changes in any significant fashion
• Boivin et al (1996) found that night shift workers experience a circadian ‘trough’ of decreased alertness during their shifts, making accidents more likely
• Tilley and Wilkinson (1982) found that shift workers had reduced sleep quality when attempting to sleep during the day