Biological rhythms

• What are biological rhythms?

  Recurring patterns within an organism that regulate the body + maintain homeostasis
• What are the 3 types of biological rhythm?

  • Infradian
  • Circadian
  • Ultradian
• What is an infradian rhythm?

  • Takes more than 24 hours to complete (days, weeks, months, etc) = occurs less than once a day
  • e.g. menstrual cycle -> 28 days
  • Entrained by exogenous zeitgebers -> seems that light, social cues + odours can affect it
  • Driven by endogenous pacemakers
• What is a circadian rhythm?

  • Takes 24 hours to complete = once per day
  • e.g. sleep/wake cycle -> humans = diurnal = increased drive to sleep at night + be awake in the day (synchronised by SCN)
  • Regulated by endogenous pacemakers
  • Can be entrained by exogenous zeitgebers
• What is an ultradian rhythm?

  • Takes less than 24 hours -> occurs multiple times a day
  • e.g. sleep cycle -> occurs 4-6 times during night
  • Regulated by endogenous pacemakers
• What affects biological rhythms?

  • Endogenous pacemakers
  • Exogenous zeitgebers
• What are endogenous pacemakers?

  • Biological mechanisms within organism -> without them, other body clocks are unable to maintain circadian rhythms
  • Enable organism to keep pace with changing environmental factors
  • Suprachiasmatic nuclei (SCN) = master circadian rhythm
  • Tiny bundle of nerve cells in hypothalamus -> built-in circadian rhythm = only needs resetting when external light levels change
  • Still work without exogenous zeitgebers, just not as well
• How does the SCN synchronise the sleep/wake cycle?

  • Optic chiasm detects light changes through action of protein melanopsin -> sends signals to SCN about light levels
  • SCN uses signals to tell pineal gland when to produce melatonin
  • = clear interaction between endogenous pacemaker of SCN/pineal gland + exogenous zeitgeber of light seen in sleep/wake cycle
• What are exogenous zeitgebers?

  • External factors which affect our biology, e.g. temperature
  • Non-biological factors too, e.g. social cues
  • Environmental cues that entrain endogenous pacemakers, e.g. circadian rhythm = 25 hours if left free-running
• What happens in the sleep cycle?

  • Re-occurs 4-6 times during the night -> 90 mins average
  • 5 stages
  • Stages 1 + 2 = light sleep
  • Stages 3 + 4 = deep sleep (SWS + NREM)
  • Stage 5 = REM