Biological Rhythms

• what is a biological rhythm?

  a rhythm is something repeating regularly

  - living things are governed by a variety of rhythms controlled by internal mechanisms and external factors
• what is circadian rhythms?

  daily rhythm that happens once every 24 hours

  - sleep/ wake cycle
• what is ultradian rhythms?


  a type of biological rhythm with a frequency of more than one cycle in 24 hours

  - stages of sleep
• what is infradian rhythms?

  a type of biological rhythm with a frequency of less than on cycle in 24 hours

  - menstrual cycle
• why is the sleep wake cycle 24 hours?

  - it is governed by an internal body clock - endogenous pacemakers

  - it is influenced by external factors such as the light/dark cycle - exogenous zeitgebers
• what are endogenous pacemakers?

  - these are internal biological clocks that determine when we sleep and wake up

  - does it work alone or does it need external cues?

  - this has been investigated by removing people from normal dark/ light cycle and normal cues to see what happens to their sleep/ wake cycle

  - will their biological clock still make them wake up and go to bed at the same time as usual?
• what are exogenous zeitgebers?

  - external cues that entrain the sleep/wake cycle

  - light, social cues, meal time
• what does Michael Siffre case study suggest?


  - conducted a series of studies on himself, in which he was isolated in a cave for up to six months

  - internal body clock set to 25 hours
• what does Aschoff and Wever suggest?


  - had participants spend several weeks in WW2 bunker

  - in both cases the participants settled down into approximately a 25 hour cycle

  - which nearly corresponds to a 24 hour day/night cycle. But not quite

  - suggesting the importance of an internal biological clock but showing that external factors play the same role in the usual 24 sleep/wake cycle
• what is entrainment?


  this is the process where external cues e.g. light, keep the internal clock in line with the 24 hour clock
• what is the free running body clock?

  this is the body's internal sleep/ wake cycle without external cues - around 25 hours
• what did Folkyard et al. suggest?

  - wanted to see if external cues could be used to override the internal clock

  - isolated 12 people for three weeks and instructed them to go to bed when the clock said 23:45 and get up at 7:45 each day

  - however the clock was gradually tampered with so that only 22 hours had passed instead of 24

  - to begin with the participants circadian rhythms were synchronised with the clock but, as the clock quickened the rhythms did not

  - this suggests that circadian rhythms are not easily re-set by external cues so supports endogenous pacemakers
• why is the wake/sleep circadian cycle just over 24 hours?

  - most research suggests our natural sleep/wake rhythm is approximately 24.9 hours

  - a more likely explanation is that the extra 0.9 hours is adaptive because it allows us to adapt to stay up later if we need to. It we need to. It is built in flexibility
• what two mechanisms control internal rhythms?

  - the suprachiasmatic nucleus

  - the pineal gland and melatonin
• what is the pineal gland?

  - the pineal gland is where melatonin is produced - the hormone associated with sleep

  - melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness

  - in humans the pineal gland is controlled by the SCN
• what is the suprachiasmatic nucleus?

  - mammals have a SCN. This is a small group of cells in the hypothalamus (area of the brain located just behind the eye)

  - the SCN regulates the production of melatonin in the pineal gland via a connecting pathway

  - another pathway connect the retina of each eye to the SCN - allowing the amount of light to influence the activity of SCN neurones and indirectly affects the release of melatonin
• what is DeCoursey et al study?

  - destroyed the SCN in the brains of 30 chipmunks

  - the sleep / wake cycle of the chipmonks disappeared nd by the end of the study most had been eaten by predators (suggesting they were asleep when they should have been awake)

  - this shows that SCN is vital for sleep/wake cycle so support endogenous pacemakers and shows without SCN exogenous zeitgebers have nothing to effect
• what is the importance of light as an exogenous zeitgeber?

  - light is the most important zeigeber in humans

  - light can reset the SCN through the eyes
• what is Campbell and Murphy research?

  - showed that shining a light on the back o the knee can shift circadian rhythms

  - shifted sleep/wake cycle by 3 hours

  - shows cells throughout the body contain proteins sensitive to light

  - light is important exogenous zeitgebers that can affect body clock
• what is practical application of sleep wake cycle?

  - shift work -- accidents + economic implications

  - jet lag

  - train babies to sleep