Circadian rhythms

• Take just 24 hours e.g sleep-wake cycle

  Circadian rhythms are needed to balance behaviour and body states to environmental changes.
• Examples of Circadian rhythms

  The sleep-wake cycle

  Heart rate,metabolic rate,breathing rate and body temp all have maximum values in late afternoon and minimum values in early morning

  Hormone concentrations also vary in the body over the course of the day
• Sleep-wake cycle

  Light (EZ’s) and dark are external signals that determine when we feel the need to sleep and wake up

  Circadian rhythm also dips, Sleepiness in these dips can be more intense when sleep-deprived, less intense when well rested 

  Circadian system (EP) keeps us awake as long as there is daylight (EZ) prompting us to sleep when it’s dark
• There is research for circadian rhythms from Siffre’s cave study. Siffre lived underground,no daylight,clocks or radio, He woke,ate and slept when he felt like it. The only influence on his behaviour was his internal body clock - his circadian rhythm. On his second occasion,he spent 6 months in a cave,and his natural circadian rhythm settled to an average of 25 hours. This supports that a typical circadian rhythm is around 24 hours,however,it shows that light may not be the main EZ in circadian rhythm.
• There is also research support from Hughes’s Antarctic Summer study. Hughes 1977 tested circadian hormone release. At the end of the Antarctic summer,cortisol levels followed a similar pattern,highest as p’s awok and lowest before they went to bed. However,after 3 months of continuous darkness this pattern had changed with peak levels of cortisol now being at noon. Hughes’ research supports that light is an extremely important EZ in the regulation of circadian rhythms.
• There are supporting findings of circadian rhythms from Anima research. Ralph et al,rats transplanted with a mutant hamsters (20 hour circadian rhythm) SCN,this changed to 20 hours for normal hamsters. Morgan et al,removing SCN from hamsters stopped their circadian rhythm so they slept and awoke whenever. Whilst such research may be challenged for lacking the ability to extrapolate animal research onto humans,there is little reason to believe that human findings would present much differently. Therefore, showing the importance of the SCN as the main EP in the circadian sleep-wake cycle.
• Luce & Segal’s Arctic Circle study provides contradictory research.

  Even in the Summer, where the sun does not set at all, the people living in the Arctic Circle were able to sleep for 7 hours a night. 

  So perhaps circadian rhythms do not heavily rely on daylight to work with the SCN, to create a sleep-wake cycle.
• knowledge of circadian rhythms has practical applications. Chronotherapeutics is the study of how timing affects drug treatments. The specific time a drug is taken is very important,for example the risk of  heart attack is greatest in the morning. so,chronotherapeutic medications have been developed,these can be administered before the patient goes to bed but drug is not released until the vulnerable period. This research support shows that circadian systems support our health on a wider scale than only the sleep-wake cycle. Therefore,  research into circadian rhythms has external validity.
• Circadian Sleep-wake cycle

  7am sun rises 

  Retinal cells pick up sunlight,release protein called melanopsin

  Signals SCN (EP)

  SCN triggers pineal gland

  Pineal gland stops production of melatonin

  Wake up

  Sun sets (EZ)

  Retinal cells pick this up release melanopsin

  SCN alerted

  Triggers pineal gland

  Melatonin released

  Sleep - cycle starts again at 7 am