Biological rhythms

• Explain what is meant by a biological rhythm.


  A biological rhythm is a change in body processes/behaviour in response to cyclical changes inside and outside the body. There are three types of biological rhythms: Circadian, Ultradian and Infradian rhythms.
• Using an example, explain what is meant by an ultradian rhythm.


  Ultradian rhythms last less than 24 hours.

  An example are the sleep stages which refer to a cycle from light sleep to deep sleep to REM sleep.

  These stages of sleep can be characterised by different brain waves e.g. light sleep is characterised by alpha waves, deep sleep by theta and delta waves and REM sleep by beta waves.

  One cycle lasts 90-100 minutes.

  We also get through approximately 4-5 cycles in one night.
• Using an example, explain what is meant by an infradian rhythm.


  Infradian rhythms last longer than 24 hours.

  An example is the menstrual cycle which refers to a period of hormonal changes and the production and release of eggs.This is because the average menstrual cycle lasts 28 days.

  The pituitary gland releases hormones that regulate the ovaries and levels of oestrogen but external factors

  such as pheromones or stress can also affect a woman’s menstrual cycle.
• Using an example, explain what is meant by a circadian rhythm.


  Circadian rhythms last 24 hours.

  An example is the sleep/wake cycle.

  Involves an approximate period of 8 hours asleep and 16 hours awake.

  This is because light causes the pineal gland to stop secreting melatonin, making somebody stay awake. Meanwhile, no light will cause the pineal gland to

  start secreting melatonin, making somebody fall asleep.
• Explain what is meant by endogenous pacemakers. 


  Endogenous pacemakers = Internal body clocks (e.g. the SCN and pineal gland) that play a main role in regulating biological rhythms.
• Explain what is meant by exogenous zeitgebers.

  Exogenous zeitgebers = External factors, such as light, that synchronise biological rhythms with the outside world.
• Outline the role of endogenous pacemakers and exogenous zeitgebers on the sleep/wake cycle.


  The presence of light (an exogenous zeitgeber) will result in the SCN instructing the pineal gland (endogenous pacemakers) to stop secreting melatonin. Melatonin induces tiredness so low levels of it means we stay awake in the day time.

  However, no light at night results in the SCN instructing the pineal gland to secrete melatonin. This causes feelings of tiredness, meaning we fall asleep.
• Evaluate research into ultradian rhythms: supporting evidence.


  They found that light sleep could be identified when the brain waves started to become slower and more rhythmic. Deep sleep could be identified when the brain waves became even more slower and more rhythmic. REM sleep could be identified when the person is still asleep but their brain activity is similar to that of when they are awake. This found that participants would cycle through these stages within 90-100 minutes, supporting the proposal that sleep stages are an example of an ultradian rhythm.
• Evaluate research into ultradian rhythms: findings are likely to be reliable. 


  This is because evidence into ultradian rhythms such as the sleep stages have come from brain scanning techniques (such as EEG) in highly controlled laboratory environments. This is a strength because such research allows for easy replication to check how reliable the findings are. Indeed, replications have found consistent results, suggesting research findings into sleep stages as an example of an ultradian rhythm are reliable.
• Evaluate research into ultradian rhythms: findings are likely to be valid. 


  This is because the research has relied on EEG recordings that are a direct measure of neuronal activity so have high temporal resolution. This is a strength because it means they can accurately detect the onset and timing of brain activity to give an accurate picture on how sleep stages meet the duration criteria of an ultradian rhythm.
• Evaluate research into infradian rhythms: supporting evidence.


  A researcher found that female menstrual cycles could become synchronised with other females through odour exposure. This is because when sweat samples from one group of women were rubbed onto the upper lip of another group, most of their menstrual cycles synchronised. This is a strength because it supports the proposal that infradian rhythms such as the menstrual cycle can be regulated by exogenous zeitgebers such as pheromones.
• Evaluate research into infradian rhythms: supporting evidence.


  One study involved a woman spending months in a cave to remove the presence of outside light. They found that her menstrual cycle shortened from 28 days to 25 days. This is a strength because it supports the proposal that exogenous zeitgebers such as light can affect infradian rhythms such as the menstrual cycle.
• Evaluate research into infradian rhythms: methodological flaws. 


  For example, stress, changes in diet, exercise etc. can lead to changes in a women’s menstrual cycle. This is a limitation because it suggests that that there are many other factors other than endogenous pacemakers and exogenous zeitgebers that can lead to a change in a woman’s menstrual cycle. In addition, this research typically involved small samples of women which questions the generalisability of the conclusions.
• Evaluate the role of endogenous pacemakers and exogenous zeitgebers : supporting evidence.


  A researcher created a genetically mutated hamster that had an SCN that lasted 20 hours rather than the usual 24 hours. They transplanted SCN cells from the mutated hamster to the normal hamster and found that the hamsters sleep/wake changed from 24 hours to 20 hours. This is a strength because it shows how the SCN (an endogenous pacemaker) governs the length of this circadian sleep/wake cycle.
• Evaluate the role of endogenous pacemakers and exogenous zeitgebers :supporting evidence. 


  The majority of blind people who still have some light perception have normal circadian rhythms whereas those without any light perception show abnormal circadian rhythms. This is a strength because it shows that exogenous zeitgebers such as light help to regulate the sleep/wake cycle.
• Evaluate the role of endogenous pacemakers and exogenous zeitgebers: supporting evidence. 


  A researcher spent long periods of time living in a cave where there were no exogenous zeitgebers to guide his sleep/wake cycle. The only thing influencing his sleep/wake cycle was his endogenous pacemakers. The researcher found that his sleep/wake cycle settled down to just over 24 hours but could change to as much as 48 hours. This is a strength because it shows how endogenous pacemakers play a main role in regulating the sleep/wake cycle but the absence of exogenous zeitgebers could influence this.
• Evaluate the role of endogenous pacemakers and exogenous zeitgebers : practical applications. 


  Jet lag is found to occur when exogenous zeitgebers are no longer synchronised with endogenous pacemakers, causing a disruption in the sleep/wake cycle. This is a strength because jet lag can result in memory problems and fatigue, so research into this area has resulted in health benefits by identifying ways of reducing jet lag.