Circadian rhythm - Wikipedia, the free encyclopedia: "Biological markers
The classic phase markers for measuring the timing of a mammal's circadian rhythm are:
melatonin secretion by the pineal gland
core body temperature[30]
plasma level of cortisol.[31]
For temperature studies, subjects must remain awake but calm and semi-reclined in near darkness while their rectal temperatures are taken continuously. The average human adult's temperature reaches its minimum at about 05:00 (5 a.m.), about two hours before habitual wake time[citation needed], though variation is great among normal chronotypes.
Melatonin is absent from the system or undetectably low during daytime. Its onset in dim light, dim-light melatonin onset (DLMO), at about 21:00 (9 p.m.) can be measured in the blood or the saliva. Its major metabolite can also be measured in morning urine. Both DLMO and the midpoint (in time) of the presence of the hormone in the blood or saliva have been used as circadian markers"
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