biological rhythms

• define endogenous pacemakers

  internal body clocks that regulate biological rhythms
• define exogenous zeitgebers

  external cues that may affect biological rhythms
• what's the main endogenous pacemaker?

  SCN (suprachiasmatic nucleus)
• outline the role of the SCN

  located in hypothalamus

  • receives information about light levels via optic nerve
  • regulates secretion of melatonin in the pineal gland
• what did DeCoursey et al find out about the SCN?

  destroyed SCN connections of 30 chipmunks

  • observed for 80 days in natural habitat
  • sleep/wake cycle disappeared
  • majority were killed by predators
• what did Ralph et al find out about the SCN?

  bred "mutant" hamsters with a 20 hours sleep/wake cycle

  • transplanted SCN cells into "normal" hamsters
  • second group defaulted to 20 hours
• outline the link between the SCN and pineal gland

  • SCN passes information on day length and light to pineal gland
  • gland increases production of melatonin which induces sleep
  • melatonin is causal factor of seasonal affective disorder
• outline the role of exogenous zeitgebers

  external factors that reset biological clocks

  • follow a process of entrainment
• what are two main exogenous zeitgebers?

  1. light
  2. social cues
• how is light an exogenous zeitgeber?

  indirect influence on body processes that control functions

  • e.g. hormone secretion and blood circulation
• what did Campbell & Murphy find out about light?

  light may be detected by skin receptor sites even if not received by eyes

  • 15 participants work at various times
  • light pad shone on back of knees
  • usual sleep/wake cycle of up to 3 hours
• what did Vetter et al find out about light?

  • one group in "warm" artificial light and other had "blue-enriched" light for 5 weeks
  • blue light synchronised rhythms to office hours
  • warm light synchronised rhythms with natural light
• what did Touitou find out about light?

  • LED bulbs enriched with blue light disrupts circadian clock
  • suppresses melatonin secretion and disrupts circadian rhythms
  • adolescents sleep becomes irregular, shortened and delayed
• what did Aschoff find out about social cues?

  individuals can compensate for absence of light zeitgebers by responding to social zeitgebers

  • circadian rhythms of air travellers adjusted quicker if they went outside more at destination
• one weakness of pacemakers and zeitgebers

  numerous circadian rhythms

  • Damiola et al - changing feed patterns alter liver rhythms but SCN was unaffected
  • peripheral oscillators in adrenal gland, lungs and liver
  • other complex influences
• second weakness of pacemakers and zeitgebers

  ethical issues

  • DeCoursey et al - animals exposed to harm and risk when placed into natural habitat
  • lacks generalisability and unethical
• third weakness of pacemakers and zeitgebers

  overexaggeration of exogenous zeitgebers

  • Miles et al - young blind man had 24.9 hours circadian rhythm
  • exposed to social cues so had to take sedatives at night and stimulants in morning
  • little bearing on internal rhythm
• what is a biological rhythm?

  distinct patterns of changes in biological activity that conform to cyclical time periods
• what is a circadian rhythm?

  type of biological rhythm that's subject to a 24-hour cycle which regulates body processes
• outline circadian rhythms

  governed by endogenous pacemakers and exogenous zeitgebers

  • last around 24 hours
• what are the two types of circadian rhythms?

  1. ultradian - occur many times during the day
  2. infraidan - takes longer than a day to complete
• outline the sleep/wake cycle as a type of circadian rhythm

  dictates when we should sleep and be awake though external signals

  • dips and rises throughout the day
  • determined through homeostatic control
  • "free-running" as maintains for 24 - 25 hours even in absence of external cues
• what did Siffre's cave study say about the sleep/wake cycle?

  • caveman spent 7 months underground
  • deprived of natural light and sound
  • biological rhythms settled to 25 hours
  • fell asleep and woke up on a regular schedule
• what did Aschoff and Wever's study say about the sleep/wake cycle?

  • group spent 4 weeks in WWII bunker
  • deprived of natural sunlight
  • all but one had circadian rhythm between 24 and 25 hours
  • can be slightly longer but entrained by exogenous zeitgebers
• what did Folkard et al's study say about the sleep/wake cycle?

  • 12 people lived in cave for 3 weeks
  • gradually sped up the clock so a 24 hour day lasted 22 hours
  • 1 participant adjusted to new rhythm
  • free-running rhythm can be overridden by external changes
• outline the core body temperature as a type of circadian rhythm 


  • sleep occurs when core temperature begins to drop
  • body temperature rises which promotes alertness
  • drop in afternoon explains sleepiness
• outline hormone production as a type of circadian rhythm 


  • production and release of melatonin from pineal gland follows circadian rhythm
  • peaks during hours of darkness
  • activates chemical receptors
• one strength of circadian rhythms

  practical application to shift work

  • night workers have reduced concentration in the morning
  • 3x more likely to develop heart disease due to lack of sleep
  • economic implications on worker productivity
• second strength of circadian rhythms

  practical application to drug treatment

  • effect on pharmacokinetics
  • shows when drugs are most effective
  • development of medication guidelines
• third strength of circadian rhythms

  research support

  • Siffre's cave study which showed increase in sleep/wake cycle
  • Hughes - lack of light in Antarctic affects exogenous zeitgebers
  • existence of EZ's and factors on sleep/wake cycle
• one weakness of circadian rhythms

  individual differences

  • Duffy - people prefer to wake early and go to bed early visa versa
  • cycle onset differs between person
  • lacks external validity
• second weakness of circadian rhythms

  other factors

  • Buhr - body temperature fluctuates 24 hour circadian rhythm
  • causes cells to be active or inactive
  • temperature may be more important than light
• third weakness of circadian rhythms

  unrepresentative samples

  • case studies or small groups
  • individual factors can impact results
  • lacks generalisability
• what's an ultradian rhythm?

  a rhythm that's less than 24 hours
• what are the main two ultradian rhythms?

  1. sleep stages
  2. basic rest-activity cycle
• outline sleep stages as an ultradian rhythm

  pattern of alternating REM and NREM

  • repeats about 90 to 100 minutes throughout the night
  • 4 stages of NREM sleep before 1 stage of REM sleep
  • information comes from EEG patterns
• outline the 5 sleep stages

  1 - 4 to 5% of cycle

  • muscle activity slows down

  2 - 45 to 55% of cycle

  • breathing and heart rate slows down

  3 - 4 to 6% of cycle

  • brain begins to generate slow delta waves

  4 - 12 to 15% of cycle

  • delta waves are produced and rhythmic breathing

  5 - 20 to 25% of cycle

  • dreaming occurs, faster brainwaves and heart rate increases
• outline the basic rest-activity cycle

  • 90 minute cycle during the day and sleep
  • in the day, move from state of alertness into state of physiological fatigue
• what did Kleitman find out about the basic rest-activity cycle?

  group of 33 participants and another with 9 participants

  • used EEGs to record brainwaves in sleep
  • humans concentrate for 90 minute cycles
  • end of the day, body runs out of resources so we get fatigued
• what did Ericsson find out about the basic rest-activity cycle?

  study of violinists

  • best performers practiced for 3 sessions each day
  • each session lasted no more than 90 minutes
  • continuous breaks in-between sessions