Circadian rhythms

• biological rhythms are cyclical changes in the way biological systems behave
• biological rhythms have evolved because of environment which organisms live in has cyclic changes (runs on a cycle for example day and night)
• nearly all organisms possess a biological representation of 24 hr day - often referred to as the 'body clock' as it optimises organisms physiology and behaviour to best meet varying demands of day/night cycle
• setting 'body clock' to correct time = photoentrainment
• circadian rhythms are driven by our body clocks - which are syncronised by the mater circadian pacemaker (suprachiasmatic

  nuclei - SCN) which is found in the hypothalamus
• the suprachiasmatic nuclei (pacemaker) must constantly be reset so bodies are in synchrony with outside world - light provides primary input)
• the circadian rhythm coordinates world with 'body clock' and determines when we should be asleep and awake
• the 3 different types of circadian rhythms

  1. sleep wake cycle
  2. core body temperature
  3. hormone production
• the sleep wake cycle is an example of circadian rhythm
• the core body temperature is a type of circadian rhythm
• hormone production is a type of circadian rhythm
• sleep wake cycle - circadian rhythm

  • light and darkness determine when we feel the need to sleep and wake up
  • out internal clock is 'free running' so it means it will maintain a cycle of about 24-25 hours even without external cues
  • experience dip and rise where our strongest sleep drives occur between 2-4 am and 1-3pm - if we have sufficient sleep the sleepiness we experience in dips are less intense
• core body temperature - circadian rhythm

  • lowest (36) is at 4:30am and highest (38) is at 6pm
  • sleep occurs when core temp starts to drop and temp starts to rise during last hours of sleep
  • drop in body temp also occurs between 2pm-4pm which may explain why some people feel sleepy
• hormone production - circadian rhythm

  • melatonin (pineal gland in brain) follows circadian rhythm where its peak levels occur during hours of darkness
  • by activating receptors in brain - melatonin encourages feelings of sleep
  • when dark - more melatonin is produced
  • when light - production of melatonin drops and person wakes
• case study to support circadian rhythm - michel suffre

  • investigated how absence of external cues could affect biological rhythms (lack of light may interrupt his circadian rhythm)
  • spent 6 months in cave with no daylight - found circadian rhythm was disrupted
  • after time in cave he became depressed and despondent at lack of freedom - short term memory, mental health and eyesight got worse
  • he concluded time is not something humans could work with and understand without any external environmental cues
  • external cues from environment can influence biological rhythm
• evaluation - research to support - hughes + antarctic participants cortisol levels - strength and limitation

  • highes studied circadian hormone release in 4 participants in antarctic station
  • in feb (end of antarctic summer) - cortisol levels followed familiar pattern reaching highest points as participants awoke and lowest when they slept
  • BUT - after 3 months of continual darkness - patterns had changed and peak levels of cortisol were at noon rather than when they awoke
  • suggests importance of light in regulation of hormone cycles
• evaluation - individual differences - limitation

  • known to be 2 key differences between individuals personal circadian rhythms
  • length of cycle - czeister et al found circadian rhythm can vary from 13-65 hr
  • cycle onset - individuals appear to be innately different in terms of when their circadian rhythms reach peak
  • differences make it hard to generalise specific times of individuals circadian rhythms worldwide - affect each outcome
• evaluation - research methodology - limitation

  • early research studies of circadian rhythms suffered from flaw when estimating ' free running' cycle
  • most studies participants were isolated from variables that may affect their circadian rhythms - clocks and radios
  • however - they were not isolated from artificial light because it was generally dim artificial light that would affect their circadian rhythm
  • therefore methods which research is undertaken using can affect the overall results of the study.
• evaluation - importance of temperature - limitation

  • the SCN responds to light entering the eye, meaning it is sensitive to cycles of day and night
  • buhr et al argues temperature controls out body clock rather than light
  • SCN transforms info about light levels into neural messages that set body temp - temp fluctuates on 24 hr circadian rhythm - buhr found fluctuations in temp set timing of cells in body causing tissues or organs to become active or inactive
  • case against light being main effector for circadian rhythm and instead temp may cause change