Environmental Entrainment of Circadian Rhythms

The term circadian rhythm, proposed by Halberg, will be used in this paper to refer to those biological rhythms which have a period length of about (circa) a day (diem). These rhythms generally persist in constant darkness at constant temperature and sometimes also in continuous light with a period which is approximately—not exactly—24 hours, hence the term circa. It is desirable to refer to the period in such conditions as the free-running-period rather than the natural period [1,2] since the period may be affected—only to a small extent, but measurably—by a number of factors which are discussed later. The basic rhythmic system is endogenous and innate. The organism inherits the basic oscillatory system, and the rhythm may be initiated by non-periodic signals. The endogenous rhythm may be entrained (see below) by exogenous factors and, as Aschoff [3] has pointed out, there may be exogenous components to the entrained rhythm.