A number of standards of performance or function on the part of the body have been accepted as indexes of its normality. Perhaps the most noteworthy illustration is found in the normal body temperature. This is something that can be readily measured with comparative accuracy, and departures from the expected figures are among the fundamental physical signs of disease. Body temperature, however, even in admittedly perfect health, is subject to characteristic slight diurnal variations that cannot be directly correlated with changes in the environment. The temperature of man reaches a maximum at about 4 or 5 p. m. (37.5 C., or 99.5 F.) and a minimum at about 3 a. m. (36.8 C., or 98.2 F.), at a time when the bodily functions are least active. It has been observed that if the habits of man are altered so that he sleeps during the day and works during the night,
DIURNAL VARIATIONS IN EFFICIENCY. JAMA. 1933;100(5):340–341. doi:10.1001/jama.1933.02740050036013
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