CIRCADIAN RHYTHMS

LAST ABOUT 24 HOURS
SLEEP WAKE CYCLE
1. External Cues/ Exogenous Zeitgebers
  1. Changes in daylight
  2. Knowing what time of the day it is
2. Internal Cues/ Endogenous Zeitgebers
  1. Free running, about 24-25 hours
    1. ASCHOFF AND WEAVER (1976) - Placed Ps in WWII bunker in absence of environmental/social time cues, found most rhythms were between 24-25 hours even though some were as long as 29. This shows free-running cycle is 24-25 hours.
3. EVALUATION
  1. CZEISLER ET AL (1999) - Altered rhythms down to 22 hours and up to 28 hours just using dim lighting
  2. In early studies P's weren't isolated from artificial light as it was thought to have no effect on the circadian rhythm
  3. CZEISLER ET AL (1999) - Found that individual circadian cycles can vary from 13 to 65 hours
  4. DUFFY ET AL (2000) - Found that morning people prefer to rise early (6am) and go to bed early (10pm) but evening people prefer to wake and go to bed later (10am and 1am)
4. MICHAEL SIFFRE
  1. SIFFRE (1999) - His natural circadian rhythm settled down to just over 24 hours but changed up to 48. But at 60 years old he was interested in the effects old age had on biological rhythms, finding his internal clock now ticked slower and his sleep patterns had changed
  2. SIFFRE (1962) - Spent 61 days underground in the southern Alps and resurfaced on the 17th September believing the date was 20 August
CORE BODY TEMPERATURE
1. Lowest at about 4:30 am (36'c) and highest at around 6.00pm (38'C)
2. Slight dip after lunch as a result of people having a siesta
3. Temperature dip is a bi-daily rhythm which is an example o an ultradian rhythm
4. EVALUATION
  1. FOLKARD ET AL (1977) - Looked at learning abilities of 12/13 year olds who had a story read to them at either 9am or 3pm, found the afternoon group (higher body temp) showed superior recall and comprehension, retaining 8% more meaningful material
  2. GUPTA (1991) - Found performance on IQ tests was best at 7pm when compared with 9am or 2pm
  3. GIESBRECHT ET AL (1993) - Lowered body temp by placing Ps in cold water and found cognitive performance decreased
  4. HORD AND THOMPSON (1983) - Tested cognitive performance in a field rather than lab setting and didn't find any correlation between core body temp and cognitive performance
  5. WRIGHT ET AL (2002) - Said higher body temp may lead to increased physiological arousal which then leads to improved cognitive performance
HORMONES
1. Cortisol, a hormone produced when we're stressed but also related to making us alert when we wake up is at its lowest at midnight and peaks at 6am which explains why if we awaken at 4am its hard to think clearly as cortisol levels are not high enough for alertness
2. Melatonin (which induces sleepiness) and growth hormone also have clear circadian rhythm and peak around midnight

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CIRCADIAN RHYTHMS

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