The third ocular photoreceptor

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Physical parameters of the circadian rhythm

The circadian rhythm is associated with variation in a range of body parameters and functions. For example, in terms of temperature, the lowest body temperature is identified at 4.30am (as low as 36.1°C) and the highest at 7pm (as high as 37.2°C). Melatonin secretion is considered to start typically at around 9pm and stop at around 7.30am. The best co-ordination function of a typical individual is identified at around 2.30pm and fastest reaction time at around 3.30pm.

Seasonal affective disorder

While there is significant complexity in the interaction of daylight within the human organism, there is general awareness of the condition known as seasonal affective disorder, or SAD. This has been recognised within various elements of historical literature and with specific reference within early western medical literature.4 While there appears to be a link between latitude and incidence of SAD - with 1.4 per cent incidence reported in Florida at 27 degrees north latitude and 12.5 per cent at 40 degrees north in New York. In Japan, however, at 40 degrees north there is an incidence of SAD of only 0.89 per cent which indicates genetic or cultural factors may be involved.