Week eight

Question 1

Endogenous clock:

Answer 1

rhythms are also controlled by an internal, or endogenous, ‘clock’. Furthermore, it has been established that in the absence of zeitgebers, the ‘free- running’ rhythms which are controlled by the internal clock tend to have cycles of about 25 hours. Under influence from zeitgebers these free-running rhythms often become adjusted so that their period is 24 hours. This adjustment of the internal clock is called entrainment. It is a necessity for otherwise there would be a continuing mismatch between our internal and external time.
The current view is that biological rhythms have both endogenous and exogenous components. The exogenous component (due to the zeitgebers) has a ‘masking effect’ on the endogenous clock. For example, lower values of body temperature are due to the internal clock, accentuated by the masking effect of (exogenous) inactivity and fasting during sleep (Minors and Waterhouse,1985). Most biological variables show higher values during the daytime and lower values during the night. If we measured body temperature, a variable used widely in experimental work because it is relatively easy to measure, we would find a fairly characteristic 24 hour curve (as shown below in the graph from the Eastman Kodak Ergonomics Group, 1986).

Question 2

Circadian rhythms

Answer 2

A large number of biological rhythms have a period that is close to 24 hours. These are commonly referred to as circadian rhythms (from the Latin: circa, about; dian, a day).

Question 3

Work-rest schedules

Answer 3

Shift work schedules, or work-rest schedules, describe the structure and pattern of required working days and hours, i.e. the pattern of alternating work and rest. Schedules are often selected on the basis of a mix of traditional, legal, economic and technological factors. There are, in addition, important ergonomic criteria that warrant consideration. These criteria are physiological and psycho-social in nature. From a physiological perspective, shift work schedules should be designed to promote good sleeping patterns, while from a psycho-social perspective the disruption of social life should be minimised. Workers should be involved in the design of shift work, so their preferences and needs are accounted for. Health monitoring should be embedded in the workplace to ensure health promotion and prevention of ill health.

Question 4

Zeitgebers

Answer 4

Biological rhythms are influenced by external, or exogenous, cues which are called zeitgebers (from the German: zeit, time; gebers, givers). These cues range from the alternation of light and day, to social behaviours including time of sleeping and diet. On the other hand, when people are completely isolated from zeitgebers for several days, biological rhythms can still be detected. It would thus appear that the rhythms are also controlled by an internal, or endogenous, ‘clock’.