circadian rhythms - biological rhythms

• Biological rhythms

  Biological rhythms are periodic activity, governed by :

  1. internal biological clocks (endogenous pacemakers)
  2. external changes in the environment (exogenous zeitgebers)

  some of these rhythms occur many times a day (ultradian rhythms) and others take more than one day to complete (infradian rhythms)
• Circadian rhythms

  The circadian rhythm lasts for 24 hours. There are several important types of circadian rhythm such as the sleep/wake cycle.
• The sleep/ wake cycle

  Exogenous zeitgebers - the fact we feel drowsy when its night time and alert during the day shows the effect of daylight.

  Endogenous pacemakers - a biological clock left to its own devices without the influence of external stimuli ,e.g. light, is called free running.

  The basic rhythm is governed by the SCN. The SCN lies just above the optic chiasm which provides information from the eyes about light. When its dark the SCN causes the pineal glad to secrete melatonin to make us sleepy and when its light, serotonin is secreted to male us alert and awake.
• Siffre study

  Siffre spent long periods in dark caves to examine the effects of free running biological rhythms. Siffre's free running circadian rhythm settled down to about 25 hours and even more importantly, he did have a regular sleep/wake cycle.
• Aschoff study

  A group of participants spent 4 weeks in ww2 bunkers which were deprived of natural light. All but one displayed a circadian rhythm between 24 and 25 hours.
• Conclusions of Siffre and Aschoff's studies

  These studies suggest that the natural sleep/wake cycle may be slightly longer than 24 hours but is entrained by exogenous zeitgebers associated with our 24 hour day.
• Folkard study

  Folkard studied a group of 12 people who lived in a dark cave for three weeks, going to bed when the clock said 11.45pm and waking up when it said 7.45 am. The researchers gradually sped up the clock to a 22 hour day. Only one participant comfortably adjusted to the new regime. This suggests the existence of a strong free running circadian rhythm not controlled by exogenous zeitgebers
• Evaluation

  One strength is its application to shift work. Shift work creates desynchronisation of biological rhythms. Boivin found shift workers experience a lapse of concentration around 6 am so accidents are more likely. Research also suggests a link between shift work and poor health, with shift workers three times more likely to develop heart disease. Thus, research into the sleep/wake cycle may have economic implications in terms of how best to manage shift work.
• Evaluation

  One limitation is that it is difficult to generalise. Studies of the sleep/wake cycle often use small groups of participants, or even single individuals. Participants may not be representative of the wider population as this limits making meaningful generalisations. Siffre observed that his internal body clock ticked much more slowly at 60 than when he was younger. This suggests that, even when the same person is involved, there are factors that may prevent general conclusions being drawn.
• Evaluation

  One strength is shifting the school day. Research shows benefits for teenagers academic and behavioural performance when lessons start later in the day. However, a later start is disruptive for parents and teachers, and teenagers may simply stay up later and still be exhausted. This suggests changing the school day may not be practical even though it may be desirable.