RHYTHMICITY has long been known to be a characteristic of many biological functions, and the mechanisms underlying biological rhythms have been of increasing interest to investigators during recent years. Some biological rhythms have been referred to as "endogenous" (self-sustained) since they are not closely correlated with any obviously rhythmic phenomena in the external environment. Others, which are obviously correlated with cyclic changes in the external environment, have been called "exogenous" (forced). Halberg 1 has used the term "circadian" ("circa"—about, "dies"—day) in referring to biological rhythms having a period of about one day.
Several studies have provided evidence that adrenal steroids are secreted on a cyclic schedule with a period of about 24 hours.2-6 By rescheduling the activities of normal subjects, it has been possible to change the hour at which adrenal steroid production rises and falls.7 All studies prior to the present one, however, have been alike
LIDDLE GW. Analysis of Circadian Rhythms in Human Adrenocortical Secretory Activity. Arch Intern Med. 1966;117(6):739-743. doi:10.1001/archinte.1966.03870120003001