Circadian Rhythm

• Biological Rhythm

  Cyclical changes in body activity
• Circadian Rhythm

  2 A type of biological rhythm in which a pattern of behaviour occurs or recurs approximately every 24 hours and which is set and reset by environmental light levels

  3 A type of biological rhythm with a duration of over 24 hours
• Endogenous Pacemaker

  Internal body clocks that regulate many of our biological rhythms
• Exogenous zeitgebers- External cues that may affect (or entrain or regulate) our biological rhythms
• Ultradian Rhythm

  A type of biological rhythm that lasts less than 24 hours (more than one cycle in 24 hours)
• Infrared Rhythm

  A type of biological rhythm with a duration of over 24 hours
• Circadian Rhythm

  •Circadian rhythms are a type of biological rhythm in which a pattern of behaviour occurs or recurs approximately every 24 hours. It is set and reset by environmental light levels.•Circadian comes from the Latin ‘circa’ meaning about and ‘diem’ meaning day.•Examples of circadian rhythms include the sleep-wake cycle (alternating states of sleeping and waking) and body temperature.
• Circadian rhythm

  Our body clocks found in all cells of the body, synchronized by the suprachiasmatic nucleus (SCN) in the hypothalamus
• Circadian pacemaker (SCN)

  • Must be constantly reset to keep body in sync with outside world
  • Light provides primary input, setting body clock through photoentrainment
• Photoentrainment

  1. Light-sensitive cells in eye detect environmental light levels

  2. Send messages to SCN via optic nerve

  3. SCN coordinates activity of entire circadian system
• Sleep-Wake Cycle

  The most familiar circadian rhythm subject to this entrainment process
• Circadian rhythm

  • Dictates when we should be sleeping
  • Dictates when we should be awake
• Morning

  1. Cells in the eye detect light

  2. Messages sent via optic nerve to SCN

  3. SCN sends messages to raise body temperature and blood pressure

  4. SCN delays release of melatonin from pineal gland
• In the morning

  • Sharpest rise in blood pressure
  • Reduction in secretion of melatonin causing us to feel awake
• Early in day

  Highest level of cortisol which makes us feel awake
• Evening

  1. SCN picks up signals of changing light from eyes

  2. SCN sends messages to ensure body temperature cools

  3. Sleep inducing hormones like melatonin are released
• In the evening

  • Body temperature begins to decrease
  • Secretion of melatonin increases, signalling time to sleep
• Circadian rhythm

  1. Dips and rises at different times of the day

  2. Strongest sleep drive usually occurs between 2am and 4am

  3. Strongest sleep drive usually occurs between 1pm and 3pm (the post-lunch dip)

  4. Small drop in body temperature between 2pm and 4pm
• Sleepiness experienced during circadian dips

  Less intense if we have had sufficient sleep, and more intense when we are sleep deprived
• Sleep and wakefulness

  Not determined by the circadian rhythm alone, but are also under homeostatic control
• Homeostatic control of sleep

  1. When we have been awake for long periods of time, the need for sleep is increasing because of the amount of energy 'used up' during wakefulness

  2. Homeostatic drive for sleep increases gradually throughout the day, reaching its maximum in the late evening when most people fall asleep
• Circadian Rhythm

  Therefore, the circadian system keeps us awake as long as there is daylight, prompting us to sleep as it becomes dark. The homeostatic system tends to make us sleepier as time goes on throughout the waking period, regardless of whether it is night or day.

  The circadian system is intolerant of any major alterations in sleep and wake schedules (e.g. through travel, shift work) because this causes the biological clock (and the internal systems that are dependent on this) to become completely out of balance.
• Sunrise

  •Light detected in eye à optic nerve à suprachiasmatic nucleus (SCN)•Body temperature increases•Blood pressure increases•Increase in cortisol (awakening hormone)•Decrease in melatonin (sleep hormone)•
• Sunset

  •Darkness detected in eye à optic nerve à suprachiasmatic nucleus (SCN)•Body temperature decreases•Decrease in cortisol (awakening hormone)•Increase in melatonin (sleep hormone)
• Eval


  •Shift work has been found to lead to desynchronization of circadian rhythms and can lead to adverse cognitive and physiological effects.•Research has shown that shift workers are three times more likely to suffer from heart disease as a result from the stress of adjusting to sleep/wake cycles.•This is a strength of the research on circadian rhythms as it has many practical implications, in terms of maintaining worker productivity and preventing accidents in the workplace.
• Eval

  •By understanding circadian rhythms and their impact on health, it can help determine the best time to administer drug treatments.•For example, the risk of heart attack is greatest in early morning so drugs can be taken at night, but not released until when they will be most effective at dusk.•This is a strength as it gives the research has useful practical applications, such as the timing of drug dosing of a whole range of medications including anti-cancer, cardiovascular and anti-epileptic drugs.
• Eval

  •There is research which supports the existence of circadian rhythms.•Michael Siffre spend two months in the caves of the southern Alps deprived of light and sound, determining that his circadian rhythm remained between 24-25 hours.•This suggests that the body’s internal clock is set 24-25 hours in the absence of external cues, and is intolerant of any major alterations to sleep and wake cycles, through processes such as shift work and jet lag. This is a strength as it shows that the concept has research support, and therefore may be valid.
• Eval

  •The Siffre study was only on one individual, and the Folkard, Wever and Aschoff studies were conducted on only a handful of people.•Consequently, it is impossible to generalise these results to whole populations of people, as there may be some individual differences in the research sample that makes these participants respond the changes the sleep/wake cycle in atypical ways.•This is a weakness as it suggests that results lack external validity, as factors such as age and gender may have significant impacts on our circadian rhythms.
• Eval

  •In the cave studies the exposure to artificial lights such as torches/phones was not controlled.•It was assumed it was only natural light that has an influence on our biological rhythms. However research by Czeisler found that dim artificial lighting could adjust the circadian rhythm between 22-28 hours.•This is a weakness as it means that the results to the original studies may lack internal validity and the sleep wake cycle may vary considerably more.