A Circadian Rhythm: the Sleep Wake Cycle

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    • Circadian rhythm is a rhythm that lasts 24 hours
    • Controlled by endogenous pacemakers, internal biological factors, and exogenous zeitgebers, external factors, such as light and dark, noise, stress and social interaction
      • Mostly controlled by the suprachiasmatic nucleus (SCN), which acts as a biological clock
        • Generates internal rhythm through protein production, but exogenous zeitgebers, particularly light and dark, synchronise this to the time zone you live in
        • SCN is a small group of cells located in the hypothalamus, just above the optic chasm, so a pathway connects it to 'light sensitive' cells in the retina of the eye
          • Light sensitive cells send message to SCN when it is dark, causing the SCN to produce a protein
            • A pathway connects the SCN to the pineal gland, so when the protein is produced the pineal gland is activated and secretes melatonin, a hormone that causes drowsiness, for a number of hours throughout the night
              • When it is light, the light sensitive cells send a message to the SCN which tops protein production, deactivating the pathway from the SCN to the pineal gland, suppressing melatonin secretion
    • When travelling across time zones the body has to adjust this rhythm to the new light and dark - this process is called entrainment
    • A Circadian Rhythm: the Sleep Wake Cycle
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        • IDA: Determinism
          • Biological pacemakers and environmental zeitgebers are both pre-determined factors which interact to establish our internal circadian rhythm which determines our sleep
            • Therefore, we have no control or free will over our sleep wake cycle
              • This is positive if an individual's circadian rhythm is effective, however, if a person struggles to sleep, the theory suggests there is little they can do about it without interventions such as melatonin drugs, as this hormone is the biological factor that controls sleep most significantly
        • Evidence
          • Morgan (1995)
            • Found that removing SCNS from hamsters resulted in circadian rhythms disappearing, and that transplanting SCNS from hamsters with mutant rhythms caused the hamsters receiving the SCN to take on these mutant rhythms
              • Study strongly supports the importance of the SCN as a sleep regulator , emphasising the SCN protein production as the most significant factor in sleep - without it the pineal gland was not activated at correct time, so secretion of melatonin did not take place when it was dark, so the sleep patterns of the hamsters were not in line with 24 hour cycle of light and dark
          • Michael Siffre and Miles et al (1977)
            • Siffre developed 25 hour sleep wake cycle when it is normally 24 hours when living in a dark cave. Miles et al found man who was blind from birth had a 24.9 hour circadian rhythm despite being exposed to external cues such as clocks and radios - could only reset circadian rhythm with sedatives and stimulants
              • Study shows importance of exogenous zeitgebers, light and dark, in adjustment of circadian rhythm, which supports theory that these could stimulate secretion of melatonin via the SCN and pineal gland. However, rhythm is still present, so this suggests biological factors play a more significant role in having rhythm, shown by the effectiveness of stimulants and sedatives. Zeitgebers only adjust this slightly from 25 hours to 24 hours - light and dark are most significant zeitgebers as radios and clocks did not improve sleep pattern
          • Mariana Figeuiro (2012)
            • 2-hour exposure to light from use of self-luminous tablets to watch films, play games, and read documents - can suppress melatonin by 22%
              • This study supports the role of exogenous zeitgeber of light - even artificial light was enough to stimulate light receptors in the retina to send a signal to the SCN, preventing protein production and activation of the pineal gland to secrete melatonin - the delay of this hormone secretion could have meant participants were not drowsy enough to sleep until later on
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          • Studies provide strong support for this biological theory because they have strong internal validity - results are measuring biological factors so they are more objective and valid
          • Studies lack population validity as they use small samples and case studies to measure biological factors - individual differences such as stress levels could also be influencing biological rhythms. Furthermore, Morgan's study is less generalizable as it uses hamsters, which could have more simple sleep wake cycles, relying more strongly on the SCN than humans which are more complex
        • Wider Evaluation: Practical applications
          • Findings from Figeuiro's study suggests parents should prevent use of tablets for children for a few hours before bed time
          • Helpful to keep room very dark before bed time
          • Melatonin drugs could be used in extreme cases
          • Travelling across time zones - could reduce jet lag by taking melatonin drugs/being in dark room
          • If trying to stay awake, eg for someone who works shifts, brighter lights, noise, stimulants could be helpful


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