Circadian Rhythms

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

The Sleep–Waking Cycle

Biological clock (link to 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.

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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)

Video clips for Lecture day

• Neuron parts and functions

• Want more? This video is from the Kahn academy