PSYA3 Circadian rhythms

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  • Circadian Rhythms
    • A01
      • Circadian rhythms are rhythms that last about 24 hours,the best known rhythms are the sleep wake cycle and core body temperature cycle.
      • Sleep / Wake cycle
        • Light enters the eyes even when they're closed, information is sent via a neural pathway to the SCN which is located in the hypothalamus
          • Information is sent via another neural pathway to the pineal gland, in the brain. Low light triggers information sent from the SCN which triggers the pineal gland to release melatonin.
            • The levels of melatonin rise when the blood levels of melatonin reach critical level which is when we fall to sleep.
              • Melatonin is released by the pineal gland in the absence of light cues, but the circadian cycle disappears if the SCN is destroyed.
        • Exogenous zeitgebers are external  time givers such as clocks, daylight and social cues.
          • Our sleep / wake cycle are entrained by social convention rather than biology.
      • Core body temperature
        • One of the best indicators of circadian rhythms. It is at its lowest around 4:30am and its highest around 6pm. But, there is a slight drop after lunch (whether you eat or not so it is not due to eating effects)
    • A02
      • Luce + Siegman
        • In a study in the acrctic, they all kept their 24 hour sleep / wake cycle despite having 24 hours of darkness. This suggests that our circadian rhythms aren't solely controlled by daylight, but things such as meal times.
      • Morgan
        • Morgan's study supports the role of the SCN. When the SCN from mutant hamsters that were bred to show a circadian rhythm of 20 hours were put into normal hamsters they showed the same rhythms as the mutant hamsters.
          • This suggests that the SCN is a major contributing factor to the regulation of circadian rhythms because if there is a problem with the SCN, circadian rhythms also change.
            • However, it was done on rats so generalisation may be problematic but it isn't possible to do on humans.
      • Schochat
        • Schochat's sleep / wake cycle found a close relationship between sleep propensity and melatonin. At 10-11pm the most melatonin was released so they felt more sleepy. Ps slept in a lab for 29 hours, for 7 minutes every 20 minutes they tried to sleep.
          • The highest propensity was found in late evening when there was high levels of melatonin released 100-129 minutes after. This suggests circadian rhythms are controlled by melatonin. Also, it has been used to treat insomniacs to make it easier to sleep.
      • Miles
        • Miles did a study on a blind man that from birth had a circadian rhythm between 24-29 hours and had to use stimulants to fall asleep / wake up. This suggests that sunlight is the most important zeitgeber in regulating circadian rhythms.
          • However, other research has shown that you don't need light to work as a zeitgeber as shining light on the back of knees is just as sufficient.
      • Folkard
        • Found that circadian variations in core body temperature has been linked to cognitive abilities. He looked at learning abilities of 12-12 year olds, the group had stories read to them at 9am or 3pm On week later, the group who had it read at 3pm recalled 8% more useful material.
          • Grupta's study on IQ supports Folkard's findings as he found performance was best at 7pm than at 9am or 2pm which suggests that cognitive ability is best when body temperature is high.
      • Research into circadian rhythms have lead to real world applications such as chrono therapeutics which is the study of how timing affects drug treatments
        • Medications that act on certain hormones may have no effect when target hormone levels are low but fully effective when they are high.


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