Circadian Rhythms

• Circadian Rhythms - consistent cyclical variations over a period of about 24 hours.

  Examples include body temperature, hormone production and the sleep wake cycle
• Research - case study of Siffre

  Spent 6 months inside a cave in Texas.

  No Zeitgebers such as natural light or time. He had contact with the outside world via telephone but had no idea what time it was. His behaviour was monitored. When he was awake, researchers put the light on and when he went to bed they turned the lights off.

  His sleep wake cycle was erratic at first but then settled into a regular cycle of about 25 hours.
• Evaluation of Siffre

  The data collected is objective, so it easy to check

  It is a case study, so generalisation to other people is unsafe. However other research show similar findings.

  The fact the that a sleep waking rhythm developed in the absence of exogenous zeitgebers suggests there are internal processes governing the sleep/waking cycle.
• Role of endogenous pacemakers and of exogenous zeitgebers

  Endogenous pacemakers - internal factors

  Exogenous zeitgebers - external timekeepers
• Suprachiamatic nucleus - SCN

  Endogenous pacemaker, a pair of tiny clusters of nerve cells in the hypothalamus which have an inbuilt circadian rhythmic firing pattern. For a number of hours the protein levels drops until it crosses a threshold where the SCN starts producing it again.
• SCN - adjusted by the exogenous zeitgeber light.

  The SCN sits just above the optic nerve. The SCN gets information on light from the optic nerve, even when our eyes are shuts special receptors in our eyes pick up light signals and carries them to the SCN

  If our endogenous clock is running low, the morning light automatically shifts the clock ahead putting the rhythm in step with the world outside.
• The pineal Gland and Melatonin - endogenous pacemaker

  Converts the neurotransmitter serotonin into the hormone melatonin.

  It contains light sensitive cells.

  When the level light falls, melatonin is produced in the pineal gland. Melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness.

  When the light levels are high, the production of melatonin in the pineal gland is inhibited and we wake up .
• Light - exogenous zeitgeber

  Light is necessary to reset the clock everyday so that the biological rhythm is coordinated with the external world.

  Light regulates the sleep/waking cycle with the pineal glad and SCN
• Research - Morgan

  Morgan removed SCN from hamsters and found that their sleep/wake circadian rhythms disappeared.

  The rhythms could be re-established by transplanted SCN cells from foetal hamsters.

  Morgan also transplanted SCN cells from mutant hamsters and found that the transplanted hamsters took on mutant rhythms.
• Evaluation - Morgan

  Its not possible to generalise from animals to humans.

  Some argue that the observed effect of removing the SCN cold be due to the trauma of the surgery or damage to other areas.

  Ethical concerns
• Practical applications of research into circadian rhythms

  Knowing about circadian rhythms is important when timing blood and urine tests and should be taken into account when taking drugs.

  Chronotherapeutics, the study of how timing affects drug treatments. It is known that taking aspirin to avoid heart attacks is most effective around 11pm.