Biological rhythms: Circadian rhythms

• What is a circadian rhythm?

  A biological pattern occurring every 24 hours
• How is the circadian rhythm set and reset?

  By environmental light levels
• What are exogenous zeitgebers?

  Environmental light levels affecting circadian rhythms
• What is a key example of a circadian rhythm?

  The sleep/wake cycle
• What are other examples of circadian rhythms?

  Homeostatic response and hormone release
• What hormone induces feelings of sleepiness?

  Melatonin
• When are the highest levels of melatonin released?

  During darkness
• What happens to body temperature during sleep?

  It begins to drop
• Sleep occurs when the body temperature begins to drop, then rises during the last hours of sleep to promote alertness.
• What drives circadian rhythms?

  An internal body clock
• What is the master pacemaker of circadian rhythms?


  The suprachiasmatic nucleus (SCN)
• How do brightness detectors in the eyes function?

  They detect changes in light levels
• What does the SCN do when it receives light signals?

  Coordinates the activity of the circadian rhythm
• What is photoentrainment?

  Light coordinating the internal body clock
• What does it mean when circadian rhythms are referred to as ‘free-running’?


  They attempt to retain a 24-hour cycle even when external cues are absent (e.g. clocks or radios).
• Circadian rhythms are intolerant of major changes in the sleep-wake cycle such as jet lag and shift work. This is because the biological clock is out of balance.
• Biological rhythms are cyclical changes in biological systems. These rhythms evolve as the environment changes.
• The suprachiasmatic nucleus is located in the hypothalamus (regulation of bodily processes/states).
• Mammals contain light-sensitive cells in their eyes which detect environmental light levels and send a signal to the SCN. This allows the SCN to coordinate the activity of the circadian system.
• Example of photoentrainment

  Sleep-wake cycle
• Strength of circadian rhythms - support from Antarctic study,

  + Hughes tested circadian hormone release in the BAS with the absence of ligh and dark.

  + Cortisol peaks and promotes alertness in the morning, dips in the evening to prepare for sleep.

  + Cortisol followed usual pattern until continuous darkness - cortisol levels peaked at noon - importance of light In maintaining circadian rhythm.
• Limitation of circadian rhythms - individual differences

  -Idea that circadian rhythms cannot be applied nomothetically.

  -Czeisler found that circadian rhythms varied between 13-65 hours.

  -Divisons of ‘morning larks’ and ‘night owls’ where some prefer to wake up early, while others wake much later.

  -A more idiographic approach is required to acknowledge variation between individuals.
• Case study: Michel Siffre (cave study)

  -Spent 2 months underground, deprived of light.

  -Maintained regular sleep schedule (25 hours) despite lack of light (supports free-running cycle).

  -Lost track of time and believed he still had a long time before he had to leave.
• Srengths of the cave study

  + Showed role of exogenous zeitgeber in maintaining a sleep/wake cycle.

  + Evidences internal biological clock (SCN).

  + Suports free-running rhythm.
• Limitations of cave study

  -Michel Siffre still had artificial light (no clocks or radios), artificial light acted as the exogenous zeitgeber. Results are invalid as light levels not fully controlled.

  -Case study (small sample), not generalisable to wider population.