Circadian Rhythms

• What is a circadian rhythm?

  A type of biological rhythm subject to a 24-hour cycle regulating body processes like sleep/wake and temperature
• How are circadian rhythms driven?

  Circadian rhythms are driven by body clocks
• What is the role of the SCN in circadian rhythms?

  The SCN is the master pacemaker that synchronizes circadian rhythms with the outside world
• How does the SCN reset itself?

  The SCN resets itself based on light detected by the eye, which sends messages about brightness levels
• What does the SCN coordinate in the circadian system?

  The SCN coordinates the activity of the entire circadian system, including the sleep/wake cycle
• What external signals determine our sleep/wake cycle?

  Light and darkness
• What is an exogenous zeitgeber?

  An external signal, such as daylight, that influences the sleep/wake cycle
• When are the strongest sleep drives typically observed?

  The strongest sleep drives are typically between 2-4 am and 1-3 pm
• What additional factor influences sleep and wakefulness besides circadian rhythms?

  Homeostatic control
• How does homeostasis affect the need for sleep?

  Homeostasis increases the need for sleep after long periods of wakefulness due to energy expenditure
• What happens to the internal circadian clock in the absence of external cues?

  The internal circadian clock maintains a free-running cycle of 24 hours
• What can cause the internal physiological system to become out of balance?

  Major alterations in the sleep/wake cycle, such as jet travel or shift work
• At what time does body temperature reach its lowest point?

  About 4:30 am
• At what time does body temperature reach its highest point?

  About 6:00 pm
• How does body temperature relate to sleep during a normal circadian rhythm?

  Sleep occurs when core temperature begins to drop, and body temperature rises during the last hour of sleep
• What explains why people may feel sleepy in the afternoon?

  A small drop in body temperature occurs between 2-4 pm
• APF(C) - Circadian rhythm study (Michel Siffre)


  A - investigate what would happen to a person’s circadian rhythms if they were cut off from all zeitgebers

  P - spent 179 days and nights in an underground cave with no lights/clocks to give any indication of what time it was - only influence was his internal body clock

  F - at first his sleep-wake patterns were erratic, after a while they settled into a 25 hour pattern rather 24 hours

  C - concluded that humans do have a natural sleep/wake cycle but without the influence of exogenous zeitgebers it naturally settles into a 25 hour pattern
• Circadian Rhythms AO3 - Research support


  • Hughes tested the circadian hormone release in four men stationed at the British Antarctic Station
  • At the end of the Antarctic summer, cortisol levels followed the familiar pattern, reaching their highest point as the participants awoke, and their lowest point as the participants went to bed
  • However, after three months of continuous darkness, this pattern had changed, with peak levels of cortisol now being at noon
  • Suggests the extremes of daylight found in Polar Regions of the world may be responsible for variations in circadian hormone release.
• Circadian Rhythms AO3 - Contradictory evidence


  • Buhr et al believe that temp controls body clock rather than light
  • Light may be the trigger, but the SCN transforms info about light levels into neural messages that set the body's temp
  • Body temp fluctuates in a circadian rhythm and small changes in body temperature can send a powerful signal to body clocks
  • Buhr et al found that these fluctuations in temp set the timing of cells in the body, and therefore cause tissues and organs to become active/inactive
  • Shows that circadian rhythms are controlled and affected by several different factors
• Circadian Rhythms AO3 - Real world application


  • Chronotherapeutics - study of how timing affects drug treatments
  • The specific time that patients take their meds is very important as it can have an impact on treatment success
  • Ex. Risk of heart attack is greatest during the early morning hours, and so these meds have been developed with a novel drug delivery system - these meds can be administered before the person goes to sleep at 10pm, but the actual drug is not released until the vulnerable period of 6am to noon
  • Increases the value of understanding CR - actively used in the medical field