Circadian Rhythms Circa: from the Latin for “about” and Diem: From the Latin for “day”

The Sleep–Waking Cycle Biological clock (link to video)video –Siffre (1975) – spent 2 months in a cave. –Wever (1979) – several months in a bunker. –25 hour cycle suggests a biological clock that needs a small amount of external correction – imp for travel, seasonal variations.

Neural basis of the biological clock –Suprachiasmatic nucleus (SCN) (sub cluster of hypothalamus). Direct link to optic nerve and pineal gland so ideal for translating external light dark cycle into hormonal fluctuations. Shows regularity to the release of melatonin (from pineal gland) a key hormone that regulates sleep.

5 How it works

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Exogenous factors—Light –Campbell and Murphy (1998): light on skin was used to shift sleep cycle to new light-dark cycle. –Miles et al. (1977) - case study of blind man. –Luce and Segal (1966) – arctic circle dwellers ignore light-dark cycle – i.e. Can be overridden.

Endogenous versus exongenous control –Jet lag –Shiftwork – 10 years shortened life expectancy. –Seasonal variation

The Consequences of Disrupting Biological Rhythms Jet lag –Changing time zones –Klein et al. (1972) Body will slowly adjust using external clues at the rate of about 1 hour per day. Some people take Melatonin supplements to adjust faster… –Melatonin and aircrew – US airforce have banned its use.

The Consequences of Disrupting Biological Rhythms Shiftwork –Usually more problematic since the effects span more than a few days… –Moore-Ede (1993) shiftworker fatigue costs $77 billion –Monk and Folkard (1983)

Circannual Rhythms Rhythms that last for about 1 year or more –Hibernation/migration– undertaken in stable lab conditions indicating that it is endogenous

Circannual Rhythms Seasonal Affective Disorder (SAD) –Severe depression during the winter months –Seasonal variation in the production of melatonin (Barlow & Durand, 1995) –More common in northern latitudes where winter days are very short (Terman, 1988)

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